That's why Microsoft won't do this. Steam users have a big investment in games on Steam, and are quite likely to switch to SteamOS, even if not every game works perfectly, rather than buying all those games again. And that would create a big Linux customer base that game studios and hardware makers would develop for, threatening Microsoft's monopoly on the desktop.
Amazing that Steam, so controversial for pushing it’s locked ecosystem at the time, is now the one pushing Linux gaming forward and a driving force _away_ from windows.
Yes, I remember the debates about truly owning a game when you bought a physical copy vs buying it on Steam. I avoided Steam for that exact reason, but then the market for games that never had a physical distro exploded, and even many/most physical games moved to a physical disk but one-time activation codes.
Once I realized there was little benefit to physical purchases anymore and the hassle of reinstalling games when I got a new machine I made the switch. But with GOG around I at least still get to mostly own the games available through them.
That would still be problematic since Valve would be at the mercy of MS, hoping they didn't change their policies after a while the way other app stores have done. Steam could still roll out SteamOS then, but it would take time. Meanwhile games might start developing integrations (and DRM) in the combined MS/Steam platform in a way that made disentangling for Valve to go their own way much more difficult.
Even if MS didn't demand a cut of the revenue, it would also probably mean Valve could no longer add whatever they wanted to Steam. If MS didn't impose some restrictions then Steam could completely allow circumvention of the MS store. Steam does sell some desktop products besides games, and presumably would want to keep that option open.
I could be wrong, but tying themselves so closely to MS would impose too many potential problems down the road and limit their ability to change or expand business models, too much risk to put their fate almost completely in Microsoft's hands.
Now, if MS purchased Valve, that might be different.
