If you want to maximize cross-platform take, use "standard" widgets. Hopefully, those will work fine and in style with overall system. Of course, certain features would be not possible to implement without going beyond standard widgets -- it is exact reason why QGraphicsView was created by Qt people in 2006, used in KDE for Plasma project and by many others.

If you would re-read what I was saying, my observations were on pointing exactly this fact: Qt has for long time been working on ways to innovate in UI beyond standard widgets for those developers who want it. There probably wasn't enough need and energy in every single Qt-based project to use those enablers but case with KDE4's Plasma shows they are still there. Multiple cases on Windows with commercial applications based on Qt or any other non-standard UI toolkit are also contributing to this view, as standard "Desktop" widgets are relatively rare in use in many Windows applications.

Who will provide "Qt standard widgets" on top of QGraphicsView by default as Qt API, whether this will really happen anytime soon (in next several years), will it be in use by someone on cross-platform market, still remains to be seen. However, I do not see any sort of panic mode activated.