QML is a good fit for fullscreen touch interfaces or video displays. Also simple game - complex ones are probably better off not using Qt at all.
QWidget is the "old" desktop UI toolkit that is good for desktop UIs.
Qt Quick is catching up with QWidgets. While still some way to go, I agree with the OP that it will align incentives for them to improve edge cases of features that exist in QWidgets but lacking in Qt Quick.
QWidgets look and act like a desktop application. QML looks and acts like a touch screen application. The two use cases are very different. A touch screen application doesn't work well on a keyboard/mouse environment. A keyboard/mouse application doesn't work well on a touch screen.
The Widgets are easier to work with for pure desktop apps… but they are limited to that purpose. Qt Quick and QML are better for pretty much any application use case that isn't the classic desktop. Especially any form of app for mobile or embedded devices.
Author here. Please show me an example for your claim.