> ... even though the user wants them separate ...
Which "the user" is this? The same user that uses GitHub's OAuth as single-sign-on credentials for other partnering sites like TravisCI, etc? If I was in the MS ecosystem and already had an identity that I could re-use (and didn't already have a GH account), I'd probably use that, just like those who login to various websites with their Google, Facebook, Amazon, or even Twitter SSO identities. I don't see how expanding SSO to Microsoft accounts would hurt anyone.
Which "the user" is this? The same user that uses GitHub's OAuth as single-sign-on credentials for other partnering sites like TravisCI, etc? If I was in the MS ecosystem and already had an identity that I could re-use (and didn't already have a GH account), I'd probably use that, just like those who login to various websites with their Google, Facebook, Amazon, or even Twitter SSO identities. I don't see how expanding SSO to Microsoft accounts would hurt anyone.