First off, if you want Ash replaced because you want someone with a better personality or a more mature series, you should probably just stop. Replacing him won't change the tone of the series, and the new replacement is still going to be our generic shonen hero. That won't change.
If you want Ash to be changed out because of growth, I can go with that. I agree that if there was a time for Ash to be finished, it was in Sinnoh. However, that's not because I think he's ready to be a champion or he's grown all he's could, it's just he grew period. In Sinnoh, Ash matured quite a bit in his rivalry with Paul, and he almost seemed older.
Then that gets thrown out with Best Wishes. He's not your normal reckless stupid, he's just silly stupid. I can take silly stupid, as he had that in the original series alongside recklessness, but he was also mischievous in the original. It's taken Ash awhile, but he slowly changed and matured from the original series to Sinnoh.
Best Wishes Ash is such a redesign and flubbed up backtracking, I pretty much consider him a different character anyway. They might as well designed a new character. Iris and Cilan are also so different in their game personalities that they might as well designed new traveling companions as well.
So originally, I was going to say that I don't think Ash should be replaced because it would be useless in producing something new. But I've just figured out I don't think he should be changed because he was already changed.