[Sylos]

The official version is that Gabe Newell thought Windows 8 to be a catastrophe, presumably due to the horrible UI. That Microsoft would single-handedly crash the PC market and take down Steam with it. So, they wanted to become less dependent on Microsoft in general.

Windows 10 is somewhat less of a catastrophe in the UI department (though there is something to be said about mostly being able to ignore the mobile UI in Windows 8), but it's still controversial with its often illegal data collection. I mean, it's probably not going to happen, because the entire world depends on it, but theoretically countries could still block the sale of Windows 10.

But yeah, I don't believe myself either that the Windows Store is not seen as a threat. When you own a platform, you can push pretty much any software to be dominant on it.