Valve doesn't have any business muscling developers into this or that platform. It's wrong if Microsoft does it, and it's wrong if Valve does it. Let the better platform win, in an open and competitive way. (I'm confident Linux would win out in such a contest, btw.)

IIRC they currently mandate Windows compatibility to launch any game on Steam. You can't publish a Mac only game for example.

If Valve are serious about launching their own platform , they're likely to apply as much muscle as they can in order to get games on there.