I am also not really a fan of the Alola team staying in Alola. At the end of every future series, Ash travels back to Palett Town and every time we have a chance to see them and maybe interact with other, non-Alolan Pokemon. Also said interactions, as minimal and irrelevant they seem to look like, actually makes Ash's reserves look like they're now all in one team.
Of course Ash can call them when a battle approaches, but then they just could have them stay in Palett Town. Now they're seperated from the rest. We may see them in PM2019 because of all the world traveling stuff, but after that? What if Ash remains for multiple more generations? Are they gonna repeat the PM2019 formula again and again? If not then the chances of seeing them again with that record of not bringing back reserves is almost zero.
I already wasn't a fan of Goodra staying in the swamps and if it really is still a Pokemon of Ash, also not one of Greninja staying with Zygarde, but now the complete Alola team? Ugh.