I've been primarily a linux user for some time, so I've gotten pretty desensitized to various applications looking vastly different on the same system. But in the last several years that's mostly gone away - Gnome knows how to change the theme for QT apps and KDE can configure GTK themes so things are actually remarkably uniform.

But then when I fire up Windows, it is just this bizarre mix of all these different UI frameworks. I guess Windows isn't the big cash cow it once was for Microsoft and maybe it's not the highest development priority anymore, but still it's remarkably off-putting and I have no idea why they would let it stay in this state.

I read they were introducing a new UI framework to unify everything (Project Reunion), but I can't help but think that will mean it's just another different-looking toolkit thrown into the mix.

With all that said, Microsoft's commitment to backwards compatibility is fairly legendary so there could be an actual technical reason some things can't be redesigned with a newer toolkit.