Targeting all platforms means the Zenimax division within Microsoft will make more money.
Targeting only Xbox means more Xbox will be sold, so Xbox division will make more money.
They promised with last-gen Xbox One, and have followed it up, that all first-party games that launch for Xbox will also launch for PC. (Originally this extended to all games, but some late releases were xbox-only.) There are no xbox-only games for Xbox Series announced, from any publisher, that are not available for PC.
Microsoft's Xbox+PC strategy is pretty much taken for granted now, "exclusivity" between Microsoft and Sony now means "just Xbox and PC" or "just Playstation (and sometimes PC)."
I think the multi-device strategy is a clear winner. The lifetime value of a console itself is probably not terribly high. After all, most game companies do not sell hardware at all and a few who have eventually gave up (Sega, Atari).
Plus, the PC, Switch, and iPad markets are largely orthogonal to the XB/PS ones. Content for the former probably doesn't compete with the latter, so it makes sense to sell in those markets. And lastly, could you imagine how Sony execs would feel if their best selling PS titles all came from MS studios? That would certainly put them in an awkward position to suddenly be financially dependent to some degree on MS.
It's not clear which one is a better deal for MS.