The problem is, people have to upgrade.Eventually, yes, but not any time soon and not necessarily to Windows 10.
Our standard in my businesses (which are relatively small and in tech industries) is still Windows 7. We don't consider Windows 10 a viable option because of the forced updates and telemetry issues, so we'd be looking at alternative platforms if Microsoft decided to dig in. As things stand, neither of those situations looks like changing any time soon; I'm guessing we're not alone, since all of our suppliers still seem to assume Windows 7 by default on new machines, so it will be interesting to see how the likes of Dell put pressure on MS to deliver something they can sell at Windows 7 nears EOL.
In our case, Apple gear would be the most obvious switch today. It has viable alternatives for most of the software we need in most areas, and crucially, it already has much better options in some of areas that aren't available on Windows. The hardware itself is relatively expensive, but it's far less expensive than making the wrong choice of platform and a lot of the applications themselves are cheaper, so it's well inside the "acceptable business expense" range.
Our view is that going cloud-everything isn't the way forward for us, so we're not particularly interested in those kinds of alternatives. Other businesses will presumably have different needs and priorities, though, and certainly the cloud-based offerings are becoming more competitive with traditional desktop software in a lot of areas for those who can meet their requirements that way and tolerate the disadvantages. If you can move your essential business software to something Web-enabled then it doesn't matter what OS you're running, or potentially even what type of device you're using.
And of course if MS do leave a huge vacuum in the mainstream desktop/laptop OS category, someone will surely move to fill it, because there's still an awful lot of money to be made there. I doubt it would be anything like any of the current Linux or BSD distributions, but as we've seen with OS X, Android, SteamOS and others, you can build a lot on top of those platforms with enough resources in the length of time we're talking about here, and certainly there are several heavyweights in tech today who have those resources if they choose to make a move.
So you might be right about people having to upgrade eventually, but I think there are likely to be a lot more viable options by that time than you're allowing for here.