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:
From that perspective alone, the QWidget class is deprecated, not KDE dying.
Youre apparently new on the forums Meet FunkSTAR, he's an anti-kde, anti-Qt troll that will refute any argument you give with the most vitriol and venom that he can muster at that moment in time. Recommendation? Ignore list