I don't see exclusives, more likely first release on XBox & PC, then later on other platforms—much like all the other exclusives work, now that Sony seem to be allowing their exclusives onto other platforms.Where will Valve and Epic stand (EDIT) if/when MS starts to make everything Gamepass/store exclusives`?
Besides, such an action would probably polarize the gaming community, so they'd be a sizeable market sympathetic to Valve, Epic etc. On top of that is the danger of approaching monopoly territory. MS have been down that road before, and won't want a repeat—they were careful to keep Apple alive over 20 years ago so they could point to them as a viable Windows competitor.
There's no question though that Game Pass is now going to be an offer most gamers can't refuse. So it's up to the other companies to compete. I'm sure they all realize gaming is moving to the cloud, and gaming hardware is going to be irrelevant soon—maybe by end of decade. It'll all be Netflix for games, same as it is for movies, TV, video, books, music.