1 - Do windows have better backwards compatibility? Not according to my experience. For example plenty of old games run better with linux+wine than with new windows versions.
2 - X is not a UI API, Qt and GTK+ are. You're thinking about the graphics server which's not any better on windows(better isolation, maybe?).
3. Why is choice a bad thing? Even if fragmentation is a drawback it's not an important one.
4. Singular UI vision is the worst - just look at how clunky are all windows UIs. They're needlessly complex and uncustomizable. They also assume that their users are stupid but they aren't. I like nice and user-friendly UIs like KDE and cinnamon and clever UIs like i3 and awesomewm. I've installed linux for many non-techie users and they quickly learned how to use their desktops - and for my surprise they started to prefer using the terminal over the software center.
5. Not sure how windows is better there. Maybe you got used to it.
6. There IS accessibility support but not perfect.
Please, go on. But in the reality the actual reasons are these:
1. Regular users(or people in general) don't like change.
2. Missing apps: adobe suite, ms office, specialized apps(at schools and offices) and the most hyped games.
3. ms has far better marketing and makes deals with governments, schools, universities and hardware vendors/sellers.