I think part of it is that the first dot com bubble significantly lessened the dominance of the operating system, as a lot of “work” shifted to the browser.
That’s how we got the browser wars, after all. For a lot of thing the OS just became a means to run a browser. You even have OSes that are pretty much built around the browser (ChromeOS, even SteamOS to an extent).
So yeah, the monopoly sucks, but it was hit pretty hard not by Competition from other OSes, but by a technology / ideology shift to the browser.
I have to admit even I see the effects of it - probably 90% of my time is spent in either Chrome, Steam, or email. The number of times I interact with Windows directly itself is … when I install it, when I look at updates, and when something goes horribly wrong. I haven’t really even used the Start menu since … I guess since 8 ruined it.