Microsoft has significantly more resources than Valve and EGS combined, not just in terms of money, they control Windows, which is an access point for the vast majority of Steam users. Microsoft can make Windows have new gaming focused features that only integrate with its store (things like Rest mode or Quick Resume etc), that would be a way to stand out from Valve's offering in a way Valve can't really combat (and if they play it right, they won't have to face much antitrust scrutiny, they aren't forced to make features that benefit their competitor).
This is why Valve is investing in Linux. They know that Microsoft will be coming after them in some way (and they already tried with UWP, look at
Gabe's statement about W8).
Additionally, with the Activision purchase, Microsoft controls Battle.net, which is a launcher that has a sizable amount of users, they're not starting from scratch here. It's only a matter of time until they decide that Elder Scrolls 6 etc etc should be launching on there instead. With Xbox downsizing they will need to extract more from PC gamers.
I think that Gamepass on Steam is not beneficial to either party (Valve gets less money than if they just sell the games, and Microsoft has to share the much smaller sub revenue with them), they may sweet talk in public about how they'd love to get it there, but there's a reason the service never landed on Steam in almost 5 years of operation now.