As I get it, Qwidget has a richer feature set than QtQuick (using QML) while both provide nearly the same functionality:
http://arstechnica.com/information-t...es-with-qt-47/
Qwidget was the original set and therefore a very large part of KDE is using that. QML was later gaining ground and has effectively replaced QWidget as the default class. Rewriting anything Qwidget into QML is a non-trivial task.
I'm not sure if all is all that bad in the long run, if only maintenance of QWidget can be kept. From the arstechnica link:
FWIW, also former Gnome-lover Canonical is in the same boat since two years, once they voluntarily jumped onto the Qt boat (
http://qt-project.org/forums/viewthread/3214):
That should mean something for the long run.
From that perspective alone, the QWidget class is deprecated, not KDE dying.