Microsoft has certainly made games based off IP they owned without the original developers. But the only examples of that I can think of is Halo, which I don't think was highly regarded.

Similarly (though not at Microsoft), when Shu Takumi took a break from the Ace Attorney franchise to do Ghost Trick, the quality of the franchise was widely regarded to have a dip as well (though now he has returned for the Great Ace Attorney Chronicles, the quality is considered to have returned). Ghost Trick was considered to be a very high quality game as well.

While IP is valuable; as an outsider to the industry, the skilled game devs seemed infinitely more bankable. I was certain that Microsoft wanted Bethesda for its quality devs, but clearly I was wrong.

You don't sack the team responsible for your best regarded game in years, if you're concerned with making good games.

You're probably right. Microsoft is probably not worried about the quality. People will still buy their favorite IP, even with a notable quality dip