I don't think this will happen or should happen for some of the reasons mentioned(ex. Quality might be impaired) but also due to the fact that it actually doesn't make a lot of sense if we consider that these regions are based off of places that are literally scattered around the world. This means if we actually traveled from l, say, Japan to the US, then to France....that would be incredibly challenging for a full grown adult, let alone a 10 year old(though, admittedly, the games are not designed to be realistic, but I digress).
What is more realistic, and I would like to see is a bigger Pokémon game based off of games in regions that are close together in real life. For example, it'd be very cool to travel between Galar and Kalos or Unova and Orre(from Colliseum on the GC) and maybe even Alola, though it may be better to wait until an additional island region game is made to pair that with Alola and make it more of a Legend of Zelda Windwaker kind of experience.
More so, what I would really love to see would be a game that combines all the regions based off of Japanese regions in real life(Kanto, Johto, Hoenn, and Sinnoh). I've seen real world comparisons to the regions of Japan that these regions are based on, and I think their comparisons make a lot of sense and could realistically be included all in one game in a way that makes logical sense in the game and in real life(ex. Start in Hoenn, train east to Kanto/Johto, travel between those two regions in the same way as before, then fly to Sinnoh). It would still be a HUGE game, but it would be incredibly fun, and if done right I'm sure it'd be appreciated by seasoned fans.
Long winded response, but just my two cents. lol think this idea is personally a ton of fun and would love to see it. :)