Skyrim has supposedly sold around 60 million copies over the span of 12 years. Let's be generous and say those sold at an average of $60 each (probably less in practice due to sales, etc.)
That's $3.6b USD in raw revenue. Pretty sick!
On the other hand, as of 2022 (Skyrim numbers are 2023), Call of Duty as a franchise had generated over $31b USD in raw revenue. 10x that of Skyrim, Bethesda's biggest title.
So in terms of scale, I don't see how making Starfield exclusive can possibly be on the same level as the idea of making the whole CoD franchise exclusive. Certainly it would be impactful but I can understand treating them differently.
Microsoft are REALLY good at playing the long game.
My disagreement would be that you're comparing an installment with a franchise. The whole of the Elder Scrolls is bigger than Skyrim, although Skyrim was the biggest standalone seller. Elder Scrolls online has I think about 750k MAUs, at $10-$15/mo.
And, I suppose more fundamentally, why would MS turn down a good chunk of $3.6b for the dev costs of making a Playstation version of Starfield?
funny thing is that if you compared the last 15 years of COD to Elder scrolls, you're still comparing some dozen COD games to Skyrim and ESO. And I guess that one mobile game? 15 years ago is still a year after Oblivion launched.