Outside of your fantasies high DPI works fine. Modern QT apps seem to pick it up fairly automatically now and GTK does indeed require a variable which could trivially be set for the user.
Your desktop relies on a wide variety of env variables to function correctly which doesn't bother you because they are set for you. This has literally worked fine for me for years. I have no idea what you think you are talking about. Wayland doesn't work AT ALL for me out of the box without ensuring some variables are set because my distro doesn't do that for me this doesn't mean Wayland is broken.