I find it pretty unlikely, both because of Gabe, but also Valve prints money and is private. They don't have anything to gain by being bought by microsoft.
There's 0 chance of this happening at long as Gabe's around.
Yeah, problem with that is that Gabe is human and won't live forever. I worry about what will happen to one of the greatest forces in keeping PC gaming not only alive, but relatively open and free from bullshit, when Gabe isn't around.
Strong chance of moving to 100% piracy forever if Gabe, or his replacement, bends the knee.