It seems Gtk has won, why don't the QT developers and supporters move to the Gtk camp and work to make it better rather then keep QT alive which just wastes efforts.
Besides, the whole "wasted effort because there's 2 projects that 2 similar things" is a fallacy.
As a Qt developer let me give you a big resounding no and ask you a question: Have YOU developed with Gtk in Windows?
Since most Qt apps on linux come from the KDE ecosystem, they often pull in a lot of KDE dependancies, which means a lot of extra packages lying around your system, creating clutter, which might bother some people. The relatively heavyweight nature of most Qt apps makes them less appropriate on systems that use just a window manager rather than a full desktop environment.
Sure, a lot of GTK apps pull in a bunch of GNOME dependancies, but there's also a large contingent of GTK apps from the lighter-weight XFCE world that don't.
Outside of linux, Qt has won pretty decisively, mostly because the GTK developers aren't interested in making GTK apps look good on Mac/Windows, so long as they run.
Although I don't actually mind as I find that once you've got one KDE app you end up installing more as they are often quite a lot better (for me) than Gnome ones - and disk space is pretty cheap these days anyway.
https://developer.ubuntu.com/en/apps/
The "out of place" argument is one I commonly used by Mac and Linux users because they want everything to look native in their OS of choice and disregard dev time (to use a different window framework for each OS) and look in every other OS.
No one can win that argument to favor THEIR own OS look over another.
Qt apps are pretty reliable across platforms and are speedy to develop so that's what should be important instead of getting an impossible native look.