I'm trying to fix the problems with X11 that supposedly justify Wayland's existence, because I have reason to believe that fixing/extending X11 would be far less painful and far easier than throwing it all away.

However, no one seems to be able to explain what is unfixable about X11. I assume in good faith that Wayland exists because there is something truly unfixable. I'd like to know what that is.

Note that I'm talking about the protocol here. People here (yourself included) point out that X.org is old and hard to maintain. This may be true, but that is a problem with the reference implementation, not the X11 protocol. Thus it doesn't in my mind justify Wayland's existence (but it does justify writing a new reference implementation).

EDIT: Here's an example -- what if someone wrote an X server that only allowed clients to render via DRI3, and by default prevented programs from receiving keyboard or mouse events intended for other programs? There would be a new protocol extension for setting and querying these blocking policies, so integrators could set more-secure default access controls without breaking compatibility. Isn't that basically what Wayland is aiming for -- client input is isolated and everything graphical happens through off-screen rendering to client-controlled GEM buffers?