What all these discussions leave out is the importance of distributed object systems (COM, Binder, ...) and related technologies that played a prominent role in GUIs post 1990. Notably COM was designed so you could create "objects" in a non-OO language like C.

I see it as an important watershed because it seems to me the reliability and particularly performance reliability went downhill around the time this technology went mainstream. I remember plain old X Windows was rock solid but once KDE and Gnome came around that were based on this technology the Linux desktop became about as reliable as the Windows desktop.