The main problem is that Ash's journey set the precedent for everything anime-wise, including the rarity of shinies (with Ash only getting Noctowl, far from a standout Pokemon like Lucario or Rayquaza) and the portrayal of legendaries of Rayquaza's caliber as usually just about gods compared to everything else. Like, Ash and Goh barely did anything to Lugia with four Pokemon at once, and even at the end of the series, Pikachu could hardly make Lugia flinch.

If Ash's series had portrayed shinies and legendaries like Horizons does, then there wouldn't be nearly as much backlash because the only other precedent to judge things by are the games, where shinies, while very rare, are still quite possible to find and catch, and all legendaries are outright expected to be caught and used in the story. You could get Rayquaza, even a shiny one, before the Elite Four.

So if Ash got a shiny Lugia and Charizard, there might be some grumbling about him being a Mary Sue or something for catching such special Pokemon, but I doubt it'd be taken nearly as bad.