How much influence did Microsoft have in this?
Sure, some people liked the Windows 8 and Windows 10 (each to their own is what I say) but trying to put bias aside I think it is fair to say that these two versions weren't exactly popular (Win8) or without controversy (Win10).
So I can't help wonder how much that helped to drive people away? I know plenty of people who got highly upset over the forced upgrade to Windows 10, some in my direct surroundings didn't even give it much of a (fair?) chance because they got freaked out and started mistrusting it. And there are a lot of people who don't necessarily use the PC for intensive things, mostly e-mail and Internet browsing. Well, that can also be easily done on a tablet and better yet: you don't have to worry about unwanted upgrades.
When I look at my direct surroundings and those who stopped using Windows then it has all been tablets or Mac. In some cases as a direct result of a (failed) upgrade to Windows 10.
As such my comment: I can't help think that Microsoft had a big influence in all this.