I live in a cave and am not a system programmer -- what's so wrong with X11 so people need to replace it with something else instead of improving it? Or, what's so wrong with anything so people need to replace it with something else instead of improving it? Let's say I'm an old man favoring improving existing stuffs instead of starting from scratch. I like debugging and wish to work as a sys programmer specialized in…
1. Security - Any program using X11 can read keystrokes, passwords, or the contents of any other window. Fixing this would break all existing X11 applications.
2. Performance - X11's client-server model doesn't work with hardware accelerated graphics, requiring hacks to get around. X11 is basically stuck with this legacy.
The ground-up re-design of X11 to fix those two issues is Wayland.