No software is bug-free. Specially not when said piece of software is literally millions of lines of code big.
And people with issues will always be way louder than the rest that runs Windows (or whatever other software you can think of) just fine.
Myself, I've been running insider builds since basically the Windows 10 launch and so far I can say that I've encountered issues rarely enough that I could count them with the fingers of one hand.
On top of that, Windows isn't Mac, where there are just a handful of different hardware configurations. There are literally millions of different configurations. And then you have to start considering whether the driver for each hardware piece is written correctly or if it's fully compatible with whatever you're running, plus whatever else may be running in kernel-mode not fucking something up, and user-installed crap.
EDIT: You could remove a lot of legacy code and clean up a bit, I suppose, but we should start thinking of getting rid of all the purely 32-bit stuff for that matter. And discarding nearly half the devices around the world that require some sort of legacy support.