I mean, if they really wanted to, they could have some regional forms on different regions and say that this is the dominant species. G-Farfetch’d could theoretically live everywhere, A-Exeggutor could also exist in Hoenn and Paldea due to the sun, A-Marowak could live in other regions with volcanoes like Hoenn or Sinnoh and so on.
This is a game where fossils can be revived (or combined), so finding regional variants in different regions wouldn’t be something too extraordinary.
If they really wanted to I suppose they could. But so far they've kept them very region specific outside of Pokemon home transfers, trades or in-game events or similar and I suspect they will keep this in-place.