I would agree that these acquisitions are meant to further bolster the value of Gamepass.Things take time the quick fix properly. The studios were not purchased until June 2018 many had games they had the release under contract. Sure they could of done a quick fix of a few paid exclusives instead and then in 2 years we would be back to square one with broken first party and the rest of the generation of little games. Microsoft is not buying another publisher for pr it is to eventually have a steady flow of content on gamepass. I would guess they would want on average a new first party game every 60-90 days by the end of the generation.
As you already stated, many of these studios had several games in production prior to being acquired by Microsoft.
Launch was a bust, and year one is bone dry as far as Microsoft 1st party exclusives are concerned. With Halo Infinite being the only notable thing on the horizon;
even then that's a big maybe as it could just as easily be a Q1 2022 title.
The prospects of all of these Microsoft funded 1st party title is interesting, but the constant speculation gets tiring imo. At some point I would like to see some sort of actual gameplay.