So that leaves how long I will have a working X server, and frankly I suspect I'll have ended up writing a display server years before that becomes an issue (whether that'll be X or Wayland or a mix, who knows, but I doubt I'll be able to resist very long).
It's just not a concern that's anywhere near the top of my list (or the top 100 of my list)
E.g. I tested, and Firefox runs fine in Weston in kiosk mode w/X as the backend.
So even when Firefox drops X, it will still run on X as long as a single compositor with an X backend is still running.
Also, as sibling comment points out, a simple solution exists in the form of rootful xwayland; we can keep a working X11 environment and just swap out the actual rendering layer.
Or put differently: "Given that X11 is going away, the server (except for Xwayland) is unmaintained," just isn't true, and even if it was it wouldn't necessarily justify the claims you make on that basis.
If X dies, I assume we’re going to end up with Javascript or Wasm frontends to everything.