Exactly what you're describing already happened when the netbook fad took hold some years ago. It's crazy obvious on Gnome but KDE apps suffered too, and the defaults to this day are still shit (optimized for form factors 2% of people use).
I feel like the solution is not necessarily to make one app do it all, but to embrace something more akin to a client-server architecture - one "app" with multiple UIs talking over the same API to the same backend logic.
Agreed, and that's kind of the point - this shouldn't be a problem to begin with, aside from the fact that "everybody" prefers to write their own shit from scratch :)
I feel like the solution is not necessarily to make one app do it all, but to embrace something more akin to a client-server architecture - one "app" with multiple UIs talking over the same API to the same backend logic.