Depends. QML sits on top of technologies and API fashions that Qt debuted before QML came along, such as the QAbstractItemModel API contract and the QObject pro

Depends. QML sits on top of technologies and API fashions that Qt debuted before QML came along, such as the QAbstractItemModel API contract and the QObject property sytem, as well as QMetaObject in general. Porting an app to Qt Quick Controls is significant work (and currently will get you lossy results as mentioned above), but there's a lot you can do to prepare while using QWidget, or quite possibly already did because it was compelling and useful to architect apps that way before already - so you can likely plain reuse a lot of your code. And the fact remains, there's no pressing need to port to Controls anyway.