-toning down abit on characters getting replaced with some acknowledgment of past traveling companions existence being made.
This could be done by starting to reuse some of older characters who proved to be popular and accepted among fans like Misty,May,Brock etc developing them more giving to fans update on them.Each of replaced characters have unfinished things and potential to bring more on table and they can provide same entertainment value like any new character could provide(if not even more).
It has already been showed several times how in order to properly advertize new games it isnt necessary to add new characters to main cast being possible to do it through giving new pokemon to current characters along with game characters appearing through arcs like HGSS characters did.
This would also strengthen connection with past regions Ash traveled through getting young kids familiarized with those who left impact on Ash character increasing continuity.
-another solution to problem lies in doing some notable steps with Ash story and career as pokemon trainer instead of being hammered down with Pikachu in strength at start of every new region with some continuity of his past achievements and progress he made over the years being better portrayed gradually developing as person and trainer.Its not like winning one pokemon league would mean end of Ash journey being testament to his skills as trainer doing big step forward.
Champion of one league tournament doesnt make you best trainer in world nor pokemon master with there existing other leagues Ash would need to win,quests like Battle Frontier where he could further advance learning more as trainer as well champion league which Sinnoh brought up and E4.
Ash has potential and things to offer for this show,writers should just better utilize it giving more substance to his development as trainer without hesitating to go step forward.No reason to flush everything Ash achieved so far down the toilet when writers could with few changes make something worthwhile about it.
Please note i dont believe any of this will happen being extremely unlikely but this seem as much better solution to me than rebooting or replacing Ash.
About Ash being reverted i think people are exaggerating with whole thing abit.Ash has been hammered but he certainly wasnt brought back to starting point showing more of maturity than he did in his starting days utilizing some knowledge in current series he gained over the years.He doesnt even act anymore like he used to in few first BW episodes actually using head when battling and reacting much differently to Iris teasing than he did when Misty was around being immature and much more naive.The characters you've mentioned have been developed, and never dregraded back in competence. Which is what sadly happens to Ash. And the two anime you mentioned are different, they're based on currently running comics that have a thought out plot and don't do a weekly formula. Pokemon just vaguely takes material from the games it's "based" on and creates "story" plots that in the end take the characters nowhere. What Luffy has gone through has taken him from point A to point B and made him grow. If Ash goes through something he gets to the destination, and may be developed but then is reverted back and acts like nothing ever happened.
If the character is going to be treated like that, and the writers expect us to accept there is a sense of continuity, they're better of having a certain protagonist for a number of series, then having a new on replace him. Because one can only maintain suspension of disbilief for so long.