I think it's pretty obvious why they're doing this now, cause not everyone has PokeBank and how else will people get Alola forms in the Switch games? By transferring them from Go.....goddamn these rumors are adding up...
I honestly expected them to add them with Gen 7.
Also, why don't niantic just add batches of pokemon, a few from Gen 4-7 every few months, like they did with Gen 3 (add in all the water types then all the desert pokemon etc) instead of doing it in generations, we'll be well into the next 2 generations of games before Go catches up.