Brock in Hoenn was okay at first but even he felt unneeded. Max helped balance his character out, and the writers were not brave enough to give May a big secondary role at first. Notice as the series went on May got more and more important and nearly stopped interacting with Brock. By BF May and Brock were nearly in the same position as Brock and Dawn, minus that contest battle they had. It was not really a bad move to give new characters more screen time then Brock. Brock has been around for years, his character was as fleshed out as possible, which is why some lost interest in him. New characters are here for nearly 4 years then they go. That is why May, Dawn, Cilan, ect are getting more fleshed out and overall better treatment then Brock, that and because they are new and from today's generation. Ash gets a free pass because they force the main game plot on him and he even steals the main boys clothes. Team Rocket are fizzling out and could very well end by this saga, or just be cut back, Brock was just the third wheel sadly. A new third character should have been in the DP trio, then they would have cut back on Ash and Dawn and it would have been fine. There was no real reason to advertise Brock a generation 1 character at all at this point and only the most loyal Brock fans will argue.
Im not sure about that,because i didnt noticed any particularly big neglect from Brocks character when May was around.He obviously still received the least(minus Max)but he had pretty good treatment even at later point in Battle Frontier.He got Bonsly,his Mudkip evolved,had some nice moments with May going on shopping with her as well participating in pokemon contest feeling closer as friends which i cant say for Dawn unfortunately.He also got to battle at Pewter gym,helped Ash,showed his skills as breeder utilizing it on many occasions along with Max idolizing him for his knowledge fulfilling each other nicely.
In Hoenn and BF i cant really complain because Brock got pretty good treatment for a sidekick.
Reason why Brock stayed for so long was probably to provide more continuity to this show with one older character along with Ash giving connection to previous regions and stability of main cast not being disrupted with complete revamps at start of each new generation like they did now with Best Wishes.
Also they needed someone to fulfill role of being older caretaker who would take care of other characters in group and pokemon,serving as un drained source of information when it comes to pokemon and their attacks,explaining to viewers some basic things from pokemon battles.
As it was showed writers when making main cast have some liberty in it not being obligated to necessarily include new ones from games to change things up a bit.I dont see any issue with older character being kept as long he is treated fine having marketing value to promote new games too(through new pokemon and events he enters).
I dont think Brock character was fully fleshed out either having still problems at home with his dis functional family,his goal of breeder was never given much attention going nowhere and while it was mostly reduced to running gag his obsession with girls provided some interesting unexplored territory in which it could have been touched upon little more.
He could very well go through crisis as character changing his career as breeder pursuing different quest which would provide new and fresh direction through which he could be developed.
In reality Brock could have been definitely developed more never being too late to fix mistakes and go forward with someone.If writers wanted they could refresh Brocks character giving him more involvement in episodes,updating his pokemon team.They could give little more attention to his breeding goal,have hm battle and participate in some events giving him more interesting aspects drawing interest from audience into this character never being too late to develop someone further reinventing some things.
Cold reality is that they probably felt how his character is fulfilling role they intended for him just fine not seeing need to increase his screen time wasting opportunities.
Introducing new character wouldnt fix anything in DP not being solution to problems being needed first to fix core of problem they are making with current character.They could gave him treatment Cilan is getting but they were making mistake of which they werent aware of, making sure now not to be repeated anymore.
Main reason why Brocks presence seemed more unnecessary in Sinnoh was lack of a bond between him and Dawn and the strong bond between Ash and Dawn that forced him out of the dynamic.