What is the story here? Creator uses it, WebEngine uses it, applications use it.
The composition infrastructure in the backingstore (now mostly in the QtOpenGL module) is tied to OpenGL. There are no plans to rework this in 6.0.
https://codereview.qt-project.org/c/qt/qtdeclarative/+/298979 present an initial, quite broken, but maybe viable, attempt to make QQuickWidget work when running with RHI and the OpenGL backend (and only then, no other backends will be functional).
Alternatively, remove QQuickWidget in dev, and so Qt 6.0.