Sometimes, additive retcons are the sort of thing that helps make a world feel more real. Otherwise, it's as if the newest region exists in isolation from everything we've seen previously.

The original version of this in Pokemon was the Johto region being Kanto's westward neighbor. Why does this new region have so many of the first game's Pokemon? It's because they're next to eachother. They even have the same Elite Four, including Lance, who is native to Blackthorn City. That explains where a Dragon type specialist came from, despite there being none in RBY.

So long as a retcon doesn't take away from what came before, it can be welcome.

Gotta retcon 'em all!