Slightly unrelated, but I wonder why the "Linux desktop maintainers" (whoever that is - and that this is even a question is probably the main problem) haven't come up yet with a minimal shared 'interface library' that's mandatory on each Linux desktop installation, and which provides a standard API (but no implementation) to create a window, get input events and probably also a set of simple UI widgets (buttons, sliders, checkboxes, lists - basically what Win32 or X11 provides).
KDE, GTK or any other concrete UI toolkit register 'drivers/plugins' with this interface library to do the actual work.
Minimal 'framework-agnostic' UI apps can then use this standard shim library and always look fully native, while applications which require the full KDE or GTK feature set can still use those frameworks directly.
Call it Common Desktop Environment or something idk...