For same reason other shows in comparison like One Piece , Naruto, Bleach etc are among most popular anime creations out there, because their continuity based story and gradual growth of established cast keep people hooked on it.
Which proved to be recipe for success and explains Ash long stay. Not to mention Ash and
Pikachu are icons of anime building its reputation and popularity around the world with millions of generations growing with them. His recognizable face created identity through which pokemon became known, attracting millions of new fans toward it and its merchandize.
New protagonist can't do that since he doesn't have popularity and charisma build around himself like long stay characters do, making it harder to spread out pokemon name globally through someone who is completely
unknown lacking reputation and familiarity through which people could associate this show to.
People want to follow growth of already experienced character broadening on his development, hence explaining why so many fans out there wants to see him grow beyond winning league such as challenging E4, going through special harder challenges doing bigger steps forward. No one is interested in following chopped alternate versions of anime delivering cheap knock of from Ash going through same basic growth all over again until he wins league, rinse and repeat after every few years never going with protagonists beyond this.
Its not about Ash in here but iconic status he and
Pikachu build around themselves and taking them out after so many years could lead to drop of interest among viewers pulling with itself drop of money.
Using BW and stagnation of Ash character as "proof" how there is nothing left to be done is logical fallacy in itself or "Cherry Picking" , because current regress of Ash character is direct consequence from game changes rather than anything else and their intention of trying to promote vibe of "fresh start". Which explains removal of contests and Brock, change of formula going back to two gym leaders traveling with main protagonist again, initial lack of pokemon etc.
People said same thing about writers not having anything else left to do when Ash won BF, but contrary to false claims writers raised up bar and developed him further in DP creating more sophisticated manner of battling coming further in league than he ever did, along with explaining how there are other steps to accomplish in order to become master aside from winning regional tournament(like champion league).
Possibilities are endless to do more with Ash growth and lack of imagination which prevails in here to take character through more meaningful , interesting pathway than what we used to see is incredible.
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