OK, so why did they buy Bethesda? What was the point if not to bolster their first party offerings? ROI doesn't have to simply be the direct sales of Bethesda games. It's enriching the Xbox/Gamepass experience. If the same can be had on PlayStation, a potential consumer wouldn't need to come to Xbox. By your logic they might as well put Halo and Gears and Forza on every platform known to man as well. More money, right?
Microsoft can bolster their 1st party and enhance Xbox/Gamepass despite putting their games on competitng platforms. In fact, they are already doing this. You can go buy Gears or Halo on Steam RIGHT NOW.
There's no path to recouping $7.5Billion through Bethesda exlusives. That's roughly Playstations entire operating profit since ps4 launched, and their exclusives sell gangbusters. The Minecraft IP was $2 Billion, and that stayed multiplat, even Minecraft Dungeons, which came out years after the acquisition.
The Gamepass experience has NEVER been about exclusives. NONE of the games on the service, aside from Forza4 are exclusive to xbox. It's been about having a huge collection of great games, including all of MS 1st party on day one, for a great price. This dynamic would still be true if and when Bethesda games land on competing platforms.
One of the biggest lessons MS has learned this gen is how to be a multiplatform developer and convert that into demand for in-house products and services. By supporting steam, they've converted some steam users into Gamepass subscribers and sell games directly to those who won't subscribe... it's a win-win. We see the same thing happening with Nintendo users. It's silly to think MS wont see the same opportunity on Playstation.
I'm not saying this will happen for all MS IP. But it makes too much sense to do this for IP that cost $7.5 Billion and already have established fanbases on other platforms.