I'm favorably disposed to QML, but for it to actually replace Qt Widgets it'd have to provide: better out of the box desktop integration, a public C++ API and a better WYSIWYG UI designer.
I tried to write a basic serial console app using QtQuick some years ago but the functionality was just too anaemic. I don't really understand how you can write a serious GUI app with QtQuick/QML. If it's just very basic controls, sure. As soon as you need something slightly complicated it totally falls down in ways that QtWidgets don't.
Also the scoping rules of QML make no sense. Children can refer to parents which totally breaks encapsulation.
I love Qt, but QML is meh at best.
Anyhow, this might be a controversial opinion, but the best experience I've had with WYSIWYG UI designers was with Netbeans Swing Editor and VS Windows Forms Editor. The IDE integration was seamless.
Yeah those are great, although I would then add classical VB, Delphi and C++ Builder as well.
I recently tried and failed. It was either doing absolute positioning or failing completely - for example dropping components onto grid would just place them into cell 0,0. There was no way to design form with mouse.
I used a lot of different UI designers in the past and all of them were heaps and bounds better.
As sibling points out NetBeans Swing designer (with Matisse/GroupLayout) is the best, along with VS WinForms (although these tend to be non-scalabale)
Sure that if you put the extra effort you can make QML look native, but it makes you wonder if desktop is in fact a goal for the framework.