Even if that’s true, the stock GNOME apps and third party GTK3/4 apps (which tend to follow GNOME design philosophy) work more smoothly under GNOME than those built with Qt or other frameworks, and so deviating makes for a materially worse experience.

And yes you can switch between multiple installed DEs and I have done so in the past, but that makes for a messy experience with many redundant apps that the user must clean up themselves. It’s a lot nicer to have just one installed.