First of all be aware of the fact, that Quick Controls 1 has become deprecated, what means that the Qt/Company has given up Qt/Quick for classic desktop applications. So be careful before riding a dead horse.
From the technical point of view: for a QPainter based framework the scene graph does not offer many advantages over what you already have with using an OpenGL based plot canvas - or Qt4/X11. And going away from QPainter does not make much sense for Qwt - or probably any other graphic package, that needs to support stuff like PDF.
About QML: it is sold as being part of the new graphic stack, but this is only because the Qt development decided not offer a C++ API for the Qt/Quick classes. In fact QML is unrelated and a widget based module like Qt/Charts can also offer a QML API . For the specific situations of Qwt I'm not sure how much it is worth. QML is only convenient as long as you can do things in a declarative ( mostly property settings and bindings ) way.
Just to mention it: being frustrated from the heaviness of QML we decided to implement the next generation of our product completely in C++. The part I'm allowed to release under a open source license is available here: https://github.com/uwerat/qskinny. Hope sooner or later it will grow into a realistic option for Qt/Quick projects, that want or need to limit the amount of QML usage.
Uwe
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