Well, that's certainly an article title isn't it. But I mean it. Microsoft, don't you dare touch Valve. You're fat enough already and too big as it is.
Valve is a private company. Microsoft can't just buy Valve, Valve would have to agree to that. Considering Valve has (for a company their size) effectively unlimited resources already, and considering that Valve's founder and leader is a known detractor of Microsoft, this is a nothing story. Microsoft will not buy Valve. This is baseless musing, like how I sometimes daydream with my wife about what we'd do if we won the lottery (which we don't play).
Of course Microsoft would love to buy Valve. Just like they would love to buy Nintendo. I'd like to buy a Lamborghini. All these things are about equivalently likely, zero likely.
We can only hope that there are people waiting in the wings at Valve who are ready to take over the reins whenever Gaben is done and carry the torch forward in the same direction.
I am pretty sure that already happened. Any bigger (or even many smaller) companies have disaster recovery plans if one of the higher ups dies or vanishes. There is a high likelihood that the next valve leader is already fixed or at least a Gremium to get to there.