Personally i do not think Ash lost personality in Sinnoh.
Ash was stil to me well Ash, same character i followed in Kanto, Orange, Johto, Hoenn etc.
He seemed more mature, firm in his beliefs and critic toward himself. Which was displayed in passionately defending his views of how pokemon should be raised when confronting Paul. Became more open minded in starting to reconsider developing new strategies taking up some ideas from others like Dawn contest moves, leading to born of all that famous counter shield.
Ash still showed plenty of characterization, funny moments and what i found quite touching scenes in Sinnoh. He was still stubborn and naive , was rash and impatient in battling others leading to his quick loses as evidenced when E4 Bertha warned him over not being observant enough or when his emotions taking best out if him in crushing defest vs Paul, He was still laidback and clueless person which hot into clash with Dawn, Angie, Barry etc.
Showed to have big appetite and impulsive behavior backed by emotional selfless and caring side of him. When he tried to earn Chimchar thrust, worked with Gligar not allowing him to have easy shortcomings to evolve without hard wok(as showed when Gary offered his razor fang, or spending whole night in bonding and training his pokemon like Buizel working like a pair.
Accordingly to that he supported and tried to comfort others like Dawn after their loses, showed care and concern for friends, got easily bored with girly and fashion stuff like he always tended to do when being in company with previous girls etc.
Now i liked Ash a lot in Hoenn too when he was more selfreliant and knowledgeable showing bigger abilitry to take care of himself, take up role of mentor to May helping her in training and preparing for contests. Coming to realization how he should stop relying on his older pokemon starting from scratch to go far through his own efforts and hard work with new pokemon.
I enjoyed in Ash a lot in his performance in original series when he was highly hotheaded, stubborn to the point of thinking he knows best not listening to anyone advice. Got cocky and overconfident at times as trainer with glory quickly hitting him in head just to be grounded real fast after quick defeats taking loses humbly wanting to improve. And lot of care compassion and devoted nature he expressed toward pokemon and his friends.
He used to be more sarcastic and brash as well, but way i look at it that kind of attitude can also be assigned to companions which surround him. For example such type of behavior Ash showed the most when Misty was atound, because her temperament, sarcastic remarks and adventurous, passionate nature in criticizing him, not letting that fame takes best of him bringing common sense and cause admiration and same spirited nature in trying to prove her wrong.
Having counterweight which teached him how to be more humble and down to earth, challenged his ego and rude nature and have him develop appreciation and care toward his friends because of having someone who pointed out his flaws, gave him motivation and somesort of challenge in working harder.
And with Misty leaving with no other character questioning his motives and giving him inducement for such reactions, its to understand how with different people and characterization, Ash relationship with them also changed because of that.
Now dont get me wrong at times i do felt how Ash lacked abit more deoth and higher diapason of emotions ti display during course of DP seeries, with things feeling static sometime.
But what i appreciated and liked about Ash character in Sinnoh was that he seemed more experienced, levelheaded as person and stronger as trainer feeling like his long travels, many close friends he made and obstacles he had to face and overcome under others guidance(in OS Misty and Brock), or coming to better understand his pokemon and starting to pass what he learned to others(like May) had left effect on his behavior and personality. Growing up and starting to act closer to how teenager if Ash ever aged to that stage would act. Which seemed rewarding to me as viewer.
That doesn't mean Ash personality and characterization "faded away", or that "his potential run its course". It was just natural way of growing Ash character up maturing and becoming more evolved as person changing perception on world around itself, pokemon and people he meets.
Lot of time maturity and personality growth is viewed as "bad thing", not being recognized and appreciated how change in protagonist characterization is needed. To make for compelling and relatable character, who will show new quirks, behavior and dimension to his personality as time passes on(personality traits, passions and different kind of demeanor we as viewer werent saware of how they exist within character until they got explored and brought up on surface fleshing out someone personality more. Directly giving out impression how character your following has learned something during all this years, that various encounters with other powerful trainers, older more knowledgeable people, various relationshiop which confronted Ash personality and beliefvs have left mark on him. Changing as trainer and person evolving because of that and preventing stagnation.
This is what happens when you take a character who was only intended to be around for 2-3 years and extend him for over 800+ episodes.
Its obvious almost all the OS characters fared the worse the longer the writers kept them in the series. I will agree Misty fared better than Brock or TR and that's likely because she didn't continue past Johto like they did.
Not sure if your familiar with other long running shows like Naruto, One Piece, Fairy Tale, Baccano etc. But they keep same characters for hundreds of episodes as well.
For example creator for One Piece manga Eiichiro Oda originally planned that seeies last for 5 years concluding character stories. Luffy, Nami, Zorro, Robin etc were originally designed to last for only 5 years. BUt mr. Oda find himsekf to enjoy too muzch in pirate adventure he wrote for,m main characters he created and grew to like wanting to continue his work and make them last for longer than its planned.
And now more than 17 years later One Piece still has exact same characters introduced in 1997, alongside new ones which joined crew later over the years increasing size of main group.
And what this shows can teach us when talking about Pokemon and main characters like Ash lasting for hujdreds of episodes?
How you can refresh older characters through new designs, pokemon, subplots and actually do sequel/build up to already established storyline bringing new angle to it. Which means more continuity, better developed character and stable cast which goes through deeper growth.
Something which is heavily appreciated among fans than endless character replacing which brings nothing new aside from face in reality, being pointless to invest in them anyway when we already know they'll leave in 3, 4 years unfinished never to be seen again.
I also asm interested to see how did Misty became worse for being in anime for 5 years? Or how Brock stay for Hoenn made him woirse(not counting DP)? Let alone ASh?
All of this characters you brought up by end of Original series and onwards like Hoenn felt like they grew and developed over time. Came closer toward achievement of their dreams and developed new sides to their characterization overcoming their struggles becoming more substantial. While keeping their fun sides and core. Things which defined them and made interesting bringing more depoth to personality. Especially in Misty or Ash case by time Hoenn started.
I also think Misty would have heavily benefit from stay in Hoenn. It has so much water based themes. Champions like Wallace and powerful water trainers like Juan, villains like Team Aqua which used water pokemon for malefic purposes centered around legendary Kyogre, possible expansion from Whirl Cup entering tournamentds which would advance her water master career, even 9th movie about Manaphy offered so much potential to develop her character more.
But to each their own i guess.
What? The Pokemon series is about the Pokemon. The games and manga always go with new protagonists every gen. The anime isn't called, "The adventures of Ash," its called Pokemon. He's obviously stayed this long because of Pikachu but after nearly 20 years its time for the series to move on. I'd wager 75% of the people who watched Kanto in 1998 aren't even watching the anime anymore, so I don't see how nostalgia is helping.
At this point it might be called that. Because Ash has became so much integral and iconic for pokemon series after 17 years that there would really be no point in replacing him for someone else. It has passed too many years to attempt that without risking long term consequences reflecting negatively on pokemon popularity and reputation whuch is already bad enough. No point in furthering problems even more.
Beside i like Ash, feel he has potential mto be done more with his story and would like that anime ends with him and all travel companions he gained over the years. Leading toward some grand finale where we get insight in each past main character lives, see them progress toward achieving their dreams and getting their 10 minutes of glory. With no new series made after that.
I mean everything has to come to an end eventually and i do not see point in new pokemon anime being made.
