KDE

They don't have to be specified in a monolithic fashion, but some things - like the input plumbing and session management examples I made - do have to be specified for for software to work when running under different compositors. FD.o basically exists because we already learned this lesson with other compat problems, and solved it without putting it in the X monolith - it's why things like ICCM and EWMH happened; there were more details than were in the existing APIs that everyone needed to agree on to make software interoperate.

Competing implementations are great, but once you have significant inertia behind competing implementations which are not compatible or at least interoperable, you've fragmented the already-small Linux market share into a maze of partially-incompatible micro-platforms. We're not going to have compositing and non-compositing, we're going to have 3ish (KDE/Qt [kde], Gnome/Gtk who aren't even doing documented protocols, and Everyone else - mostly [wlr] extensions) incompatible sets of protocols for basic functionality.

Looking at the slow bitter process to extend or replace components once implementations that rely on them exist, that's not something to count on. Remember how it took 15 years of contention to eventually transition to D-Bus after CORBA/Bonobo and DCOP? That's whats about to happen with things like the incompatible gtk and qt session management schemes. And that resolution was forced by the old HAL system using it, not the other parties involved getting their shit together of their own accord.

One place we're about to see innovation is wayland-stack-bypassing workarounds. Key remapping is currently in that category, the wayland protocols suite punted.... so instead, keyd sniffing all the HID traffic at the evdev and/or uinput layer and outputting the rule-edited streams to virtual HID devices. That one does have a certain global elegance (works on ttys!), but it's also layering violations with privileged processes.