Isn't this what Gnome/Libadwaita is trying to solve? They've been putting quite a bit of effort into it, and gnome builder, flatpak & related tech to have a proper developer stack for making GUI apps, and a community (gnome circle) to show them off.
At one time Gnome ejected a portion of its user base in order to become something that would work on tablets and phones as well as desktops - at least that seemed like the rationale. At that time it looked like there might be a market for linux devices with a UI and I don't know for sure but I think some money was spent to achieve this which is why objections by users were of so little importance.
Nowadays phones seem like a duopoly that cannot be challenged and tablets aren't very important anymore. Linux doesn't matter on the desktop because browsers are the UI and the apps run in the cloud. The whole GNOME/KDE/whatever effort is a bit moot.
Yes, both GNOME and KDE, but unfortunately never took off as many of us expected in the 2000's, and nowadays apparently tiling managers without such frameworks is what is cool.
