Well if we apply the same logic than we can only conclude that the deal wasn't to remove IP from the competition as exclusivity deal would also have cost only a small fraction of the acquisition.
Take market share (by removing IP from the competition). Financial don't make sense otherwise. Exclusivity deal cost more because you need to pay for the lost of sales on the competing platform + pay for the gamepass loss of revenues (for the game company).
No business thinks like that... Nobody is doing anything to "remove" marketshare from someone else. (well... at least successful business do not).
Everything is always forward thinking and aimed to grow THEIR own marketshare.
Obviously, the side effect of that is that the competition will most likely lose some marketshare but at the root, the idea/goal is to grow your own business.
When Microsoft is buying Bethesda/Activision, they do not care about the impact (or the extend of it) of removing this from the other plastic box out there, they care about how much value it will bring to their own offering and how this will make GamePass and their platform more attractive to customers, which is in return grow their gaming business.
I understand that those 2 are very similar :) But at the heart, it is quite different imo, and it drives several decisions a bit differently.