I still don't see how this is going to protect you from writing native code for all platforms. You can have all the abstractions you want, at some point you need to code to the native toolkit. The code that actually gets a window or button on screen, or receives (rather than processes) the input events.
In case it isn't clear: Qt Widgets (and WxWidgets) did this for you. You could literally compile a native looking and feeling app for the major platforms from an identical C++ codebase.
Qt emulates the platform L&F hence why Mac users "love it".
Also Qt doesn't do this for any mobile OS. You are expected to do it yourself with QML. The Quick Controls only cover the mostly used ones and still don't do iOS or UWP themes.
Yes you need to write platform specific code, but it makes use of the native UI, not emulating it, and after a while you have your in house framework, so it is not like you are writing from scratch every time.
This is the approach taken from Dropbox or Microsoft, for example.
In case it isn't clear: Qt Widgets (and WxWidgets) did this for you. You could literally compile a native looking and feeling app for the major platforms from an identical C++ codebase.