I wonder if the show would have been better off introducing a new protagonist for each generation instead of having the same character through all of them. That way it makes sense for the protagonist to be clueless about Pokemon and each generation can be repurposed for a new age group. It will also allow more flexibility for character development and story resolutions (Ash still hasn't won a single league?).
The only problem with this is Pikachu, but they can find some random reason to keep him present throughout the entire show.
It's too late to change things up like that, though. Ash is here to stay, for better or worse.
When other animes, including Yo-kai Watch, introduce new protagonists, it doesn't end well. Even Yo-kai Watch, which is popular, seems to have massive rating drops when it's a USApyon episode rather than a Nate episode.
Besides, it wouldn't do much. It'd be a similar protagonist with a similar goal. It'd be an aesthetic change above all else.
Besides, people overstate the issue regarding Pikachu and Ash. Pikachu only lost to Snivy at the start of Black & White because it had been severely weakened from an attack by Zekrom. It can easily be put in game terms for people to understand.
Because Pokemon is a cartoon, and cartoons are for kids, and kids can't deal with more mature themes.
You need to watch XY then. The XY series has had some dark themes. A lot of references to death and loss in it. It's just not gratuitous.
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As for the whole "Ash is still 10" crap. Really? It's a long running animated series. They seldom age their characters. Why is this an issue with Pokémon and not The Simpsons or Family Guy (granted it has aged the characters like 1 to 2 years in the 20 years since it began), or the Jetsons etc.? Besides, they reference the passage of time. If it wasn't for the start of Black & White's series calling Ash 10 again, this wouldn't be an issue as his age is never mentioned, especially considering how Ash is drawn and acts more mature these days.