Obviously, buying rights to everything is an exaggeration.
You're either deliberately ignoring the obvious point - that MS has the money to handle Valve in multiple ways - or..... well, I don't see any other options.
I mean, Gamepass was certainly a way to siphon off Valve users. They've tried to make Windows Store a bigger deal. I don't really see what else they could do to try to crush Steam unless they start getting more exclusives on Windows Store/Gamepass? People have tons of games and friends on Steam and many probably are unwilling to move to a new platform because they don't want to have split libraries and there are probably a good chunk who don't want to move simply because they have so many Steam achievements.