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