Each and every one of mentioned male companions was needed having value and useful position they played in pokemon anime.
First of all just because Ash is driving force of pokemon anime doesn't and cannot determine that each and every companion role has to be played around him. For instance i hear people saying how moment Brock stopped being Ash mentor, he "lost his role in pokemon" which is very, very wrong thing to say in my opinion.
Because writers in Hoenn expanded on his breeder career, had him catch more pokemon, get more evolutions and learn new things as breeder such as how to prepare new type of food(see pokeblock), exchange experiences with other breeders, utilize his knowledge in resolving disoutes between pokemon like Grovyle vs Tropius, Solrock and village people, agitated Magnezone and Metagross etc. Taking care of injured or sick ones such as Nuzleaf, Sharpedo or Bagon growing forward both as person and breeder.
While playing important role logistic wise taking care of Ash and co as older brother, provide supplies, give advices to May about her contest career and bring humor, sense of security and balance in main cast keepong things under control(just like he did with Misty and Ash when things escalated way out of control). Through his eccentric behavior, comedy nature of losing his head over other girls or sometime pokemon trying to use them for his own plan of gaining other girl attention(like Lotad), caring, solidary side in helping other pokemon revealing unselfish, passionate aspect he holds for pokemon himself transfering knowledge to others(like Dawn, Ash for example)etc.
And often helping Ash in training pokemon as his sparring partner in Hoenn or Sinnoh still coming up with strategies and help over his pokemon or gym battles(like vs Brawley for example, Winona, Chimchar, Treecko etc.)
Same applies to Cilan who was like navigator, older brother which due to his wise, more grown up attitide knew more about pokemon battles, strategy and events specific for Unova region than Ash was. Getting him familiar with customs, other gym leaders etc.
Bringing lot of flare, humor and tense atmosphere with his witty sophistry with Burgundy, clashing moments when getting into feud with Iris or Ash. Overdramatizing over his food, pokemon and various hobbies or showcasing vulnerable, fearful side to himself when paranormal things are in question.
All done with purpose of adding more diversity and energy in main cast, which in itself i already viewed as meritorious and important role in making things more appealing for viewers(because entertainment is often most important ingredient to catch people attention).
Or Tracey to lesser extent who got Ash and Misty more familiarized with Orange Islands archipelago, prepared food for them, was familiar with champion, gyms and how whole Orange league works. While serving as intermediator between Ash and Misty's more dynamic, hotheaded personalities by injecting calamity and at times amusing stinging through his jokes of them being in love, and feeling jeopardized/ flustered when Ash or Misty made sarcastic remarls over his subtle interest in other women.
Bringing us to important inference how; Worth of companion in pokemon series isn't tied solely to Ash role and how much their presence helps move Ash story forward.
Otherwise going by that logic May, Dawn, Iris or Misty were never needed to begin with because their stories, flaws and imperfections in most cases had absolutely nothing to do with Ash career of becoming pokemon master influencing little to none his gym quest. This especially applies to Iris, May or Dawn when Ash wasn't anymore rookie who needed other people guidance making them thius completely "pointless" if character usefulness is measured by how much he helps main star.
Being ignored that point of pokemon isn't only in following Ash journey and him establishing friendship with other characters. But also in exploring lives of other companions which join him on journey, give viewer look in his own past, motives, issues over which he has yet to pass and see him advance his own goal. Follow development of his /her personality, pokemon and subplots centered around him.
With main and primary mesage of pokemon lying in promoting friendship between Ash and other friends which accompany him on journey, build on relationship with interaction over time gaining more substance, to see each and every one of them learn new things on their voyage, overcome obstacles, work toward progressing their goals and dreams. Along with gaining experience, knowledge and gradually maturing as result of that moving story forward through their own interests.
With not everything falling down on Ash back.
Yes Misty, Iris, May or Dawn brought in general more emotion, depth and flare in characterization(in my humble opinon Misty did especially good job in that field). They received more character development, introspection of their own personalities, fears and aspirations than any male companion did. We learned more about their past, saw them battle more and advance their dreams leaving bigger impact on story.
All of this is true.
BUT, Cilan, Brock or Tracey to smaller extent played important role in pokemon anime through supporting role. Being backbone on which Ash and girls could rely when something wents wrong, made sure to look after younger members of cast to prevent them from mindlessly rushing in danger, did navigation , providing cast with supplies or help when pokemon was seriously injured or sick through battle, poison and exhaustion through preparation of potions and remedies(this especially holds truth for Brock). Being older, more experienced face which kept everything on hold, having their own pokemon, issues, insecurities in characterization. Most importantly dreams and wishes they wanted to accomplish.
Containing required appeal, humanity and uniqueness to grant them popularity and approval among fans.
Brock was only there to be there in Diamond and Pearl (what a painful 4 years THAT was, but OMGZ DP was SOOOO GEWD), but he wasn't absolutely needed past the Original Series, either: he was still good in Advanced Generation, but started going downhill in Battle Frontier even there.
Overall, yes, but only for a while-- at least Tracey didn't outlive his usefulness, though.
Clemont is the only good character in the XY Series.
I don't know, i thought Brock was handled better in BF than he was in Hoenn imo.
Brock got lot of really good focus there and genuine, emotionally packed interactions with others. He challenged May to contest battle giving his all to win Yuma heart and impression, showed lot of pride and understanding for his jounger brother Forest pairing up with Ash in tag battle to defend Pewter gym vs Team Rocket. Entered pokemon orientering, had his Mudkip evolve, caught Bonsly gaining his thrust through caretaking abilities. As such reveaing that he grew a lot as breeder over all that time he spent in traveling with Ash and exploring world. Was involved in resolving issues between other pokemon trainers pokemon like Flaffy and Ampharos, started to show interest in becoming pokemon doctor when taking care of sick nurse Joy realizing value of her job, being like early foreshadowing toward him eventually becoming one. Seducing of other girls didn't stayed just on gag actually evolving in something more like gaining simpathy from Frontier brain Pike Queen Lucy developing crush on him for a change etc.
Brock was great in Battle Frontier in my opinion, infact one of highglights in what i found to be insult to Kanto, lack of more active use of characters inherent to it which could lift up dynamic and bring new, interesting twist to story through their own goals and further development(see Misty, Sabrina, lt. Surge etc) and way it was referenced being largely static, subpar saga imo.
As for Tracey he was sadly removed before he could even live up to his role, being bad move toward his character. Never staying long enough to gain more dimension to personality and character by giving us info of his past, origins, interests etc. Or developing his own pokemon, career and plot showing to be detrimental in his general acceptance among audiemce not receiving much from short stay to invest and relate themselves to character.
In fact i don't think any character in pokemon "outlived their welcome" strictly speaking. Because for most of them regardless of how writers handled their characters, story and dreams doesn't change fact how they had potential, energy and appeal to last for longer than they did in case writers decided to make use of their unused material.
Or created fresh new obstacles, expand on unresolved aspects of ther story and develop new plots which would bring fresh life to characterization and their role doing more with character than they originally intended. Sequels can and do get made with huge success on other long running shows.
However, I think he should have left in DP. It's not his fault, it's obviously the writers, but among the other bland characters, he was especially bland. They had no idea what to do with him after just a couple dozen episodes, but I'm glad he's gone because I don't want his character to be ruined even more.
This can go both ways though. If Brock stayed writers could have do with him what they did with Cilan and now Clemont increasing his role in episodes, expand on interest and explore more on Brock character and past. In sense of introducing unresoled issues, desires and events which werent brought up until than because of situation not calling for it.
Something writers of other anime do when they feel character may start becoming turing to auduence wanting to add new twist to his story and role , building up on character.Instead of runing, fixing him up regaining lost luster and becoming eventful again.
I mean examples of Brock successors shows that they know and can do better job with male sidekicks(although Brock in OS and AG still beats them in my book), so in essence them learning on past mistakes could have been applied to Brock character too(just glimpse of his role in special with Cilan when Brock played important part in healing Cilan pokemon and climbing on Gyarados to treat his wounds revealed how staff can do better job using his new career and possibilities it provides to do something new with character).
What was the point of Brock by DP? He had already run his course by the end of Hoenn, it really was nonsensical why he kept coming back for multiple generations only for the writers to completely ignore him.
Just to point out something. Character being neglected=/=character necessarily not having anymore potential. With Brock in essence having material to do more with him than what he has ghotten in end.
In fact every replaced companion had stamina , unresolved things and plots around which viable continuation could have been made if writers actually were motivated to take them forward. That is.