Disclaimer: I'm not the authority on this nor an insider. This is just my speculative thoughts based on how a publicly run corporation would likely think in this situation.Doesn't need to be exclusive to drive game pass subs, though. If what you say is true you wont be able to buy the games at all and it would literally be GP exclusive. Which isn't happening. Why take a risky long term approach that has no gurantee to speed up GP adoption when you can start recouping right away, and still drive GP growth.
There is no risk in keeping these games exclusive. The risk is not staying on track with Game Pass adoption since that is the engine of the entire gaming division. You protect the heart and engine of your division/product. If the data starts to tell them different things later, they can always pivot. It's easier to do that than find out lack of exclusives are costing you Game Pass goals and try to reverse that course. You only get this one unique chance to sway mindshare. Consoles generations don't come along often. This may be the last time we have anything close to a traditional console transition. Even those gamers who are 95% committed to Sony next gen are more hyper aware about what's going on in the gaming industry during the transition period (and especially due to the unique situation of being locked inside). Lock the Bethesda mindshare to Xbox now. Plant that seed and maybe they bear fruits. 70% of new Xbox owners are getting Game Pass. They will try to drive new consoles aggressively until cloud gaming is viable...which won't be the case for the mainstream until the end of this generation.