The new architecture allows developers to fix one thing without accidentally breaking 3 others.

That’s an extremely bold claim, and vague, with no actual examples.

The problem is not the code per se, but that we can’t add stuff anymore that doesn’t somehow break the core protocol. The plain fact is that we’ve been tacking on things to X11 which it was never designed to do for decades and we reached a breaking point a while ago.

Stuff like multi-DPI setups are impossible to implement in X11’s single-framebuffer model; security on X11 is non-existent, but we can’t retroactively fit any kind of permissions on the protocol as that breaks X11 applications that (rightfully) assumed they could get a pixmap from the root window. There’s so much more, just take look at https://www.youtube.com/watch?v=RIctzAQOe44

Then the problem is that it’s abandoned, not that it has stagnated

By all means, feel free to start working on it!

All the people who developed Xorg for 20+ years decided that creating and working on Wayland was a better use of their time. But I’m sure you know better…

The problem isn’t that Xorg is spaghetti code (it’s pretty good for a large C project, imho). The problem is that the X11 protocol was designed to expose the capabilities of 1980s display hardware.