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.