Hmm, B2W2 is definitely different in terms of narrative to BW so they definitely need to be separate games, but if you mean the gameplay content, then yeah, BW could definitely have benefitted from the features added in B2W2.
But in regards to SM, since gen 6 was dropped in such a haphazard fashion, I'd honestly rather just leave Alola behind and move onto bigger and better things if they fail to deliver a fair variety of new Pokémon, forms, and the like. Why stick around in Alola longer than we have to if it couldn't give us the regional variations and whatever other things we wanted the first time around? That was a lot of peoples' logic when it came to skipping over a third version to XY and I think it can easily apply here. SM is going to give us access to Zygarde Complete so I'm sure a future, non-Alola game can give us access to new forms of non-gen 1 Pokémon. ...That is, if Game Freak ever decides they're worth their time to make...
It's true that we don't know all the details about Alola forms, but I think you may be overthinking their general complexity here.
Here's an official screenshot of a Pokémon Bank account connected to Pokémon X:
Notice the Pokémon with the kind of grayed-out background? Those originate from gen 7, and can't be transferred backwards into gen 6--so, the usual transfer limitations.
Here's an official screenshot of a Pokémon Bank account connected to Pokémon Sun:
Notice how both a regular ol' Vulpix and Alolan Vulpix can be sent into Sun. I think it's safe to say that this point towards Alola forms being, well, a form difference, which means they can be traded from game to game (and transferred into newer generations), but with limitations on how they are obtained. Even if we never see regional differences outside of Alola, I'm certain we'll be able to transfer and use (and probably even breed) Alola forms in future generations.
As for "Are Sinnoh forms really going to be a thing?" well, you tell me: will we ever get new Mega Evolutions?