The major weakness in their desktop OS market dominance isn't from other desktop OSes. MacOS, ChromeOS, and traditional desktop Linux distros prevent Windows from being a total monopoly, but there's no doubt that Windows has quite a bit of market power.
The real competitive threat to Windows is from people who decide to not use a desktop computer at all. Between tablets and phones, there are a lot of people who no longer feel the need to have a laptop or desktop at all, for personal use.
And on that front, Windows being shittier than phones and tablets will cause people to slow down their upgrade cycle and maybe avoid using a traditional personal computer at all.