16 Billion dollars is nowhere NEAR what Valve is worth, if we consider that Activision is worth 35 Billion.
That said, Microsoft COULD introduce Windows Updates that make Steam seen as dangerous, limit installations, throw warnings, break functionality. Let that disaster stew for a while, let Valve's engagement drain a bit as normies start to stop logging into it because Windows blocks it running on startup, they can't trust games purchased on it to run smoothly without issues....
Really run the value of Steam down using the resources they have.
THEN they make the bid.
Valve would sue the hell out of MS if they tried. And you can bet it would trigger anti-trust lawsuits. Not to mention how pissed off damn near every single publisher in the game industry would be and how many of them would also join in on a lawsuit pile on. MS would completely kill their entire gaming division if they tried that.