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.
Is the security extension from 1996, which has a section on keyboard security
and its crazy to me that this anyone can claim X11 can't be off loaded, which its been doing for decades. From all the crazy blt/pattern HW acceleration to GL/vulcan implementations to the fact that the entire server can be on the other side of a network pipe, meaning it could be anywhere, including entirely encapsulated on a graphics card/smart nic/etc.
And if your talking about the xlib serialization, that was largely fixed with XCB.
And its not like actual flaws people found couldn't be fixed.
There is a word for this.