First, being abandoned for new development is a great indication of the quality of a project.
Second, you haven't given us any arguments for your "indications of quality" regarding MATE and KDE.
Third, like Gnome, KDE has a huge legacy in FOSS, and is a great project in itself based on a top notch GUI backend. Some of its code even went on and become the basis of the modern web (KHTML -> Webkit -> Blink -> Node -> now also Edge), other tools like KDevelop, Krita, etc are among the best in class in what they do.
What are you, some teenage Linux nerd, with a "favorite" desktop to promote in flame wars?
MATE is not a GNOME project.
When GTK3 was made and the decision to build Gnome Shell was made, the MATE project was started to fork GTK2 and the old shell.
Second, you haven't given us any arguments for your "indications of quality" regarding MATE and KDE.
Third, like Gnome, KDE has a huge legacy in FOSS, and is a great project in itself based on a top notch GUI backend. Some of its code even went on and become the basis of the modern web (KHTML -> Webkit -> Blink -> Node -> now also Edge), other tools like KDevelop, Krita, etc are among the best in class in what they do.
What are you, some teenage Linux nerd, with a "favorite" desktop to promote in flame wars?