In 1996, they came up with the X11 “SECURITY” extension:

https://www.reddit.com/r/linux/comments/4w548u/what_is_up_with_the_x11_security_extension/

This is what could have (eventually) solved the security issues that we’re currently seeing with X11. Those issues are cited as one of the reasons for switching to Wayland.

That extension never took off. The person on reddit wonders why – I think it’s simple: Containers and sandboxes weren’t a thing in 1996. It hardly mattered if X11 was “insecure”. If you could run an X11 client, you probably already had access to the machine and could just do all kinds of other nasty things.

Today, sandboxing is a thing. Today, this matters.

I’ve heard so many times that “X11 is beyond fixable, it’s hopeless.” I don’t believe that. I believe that these problems are solveable with X11 and some devs have said “yeah, we could have kept working on it”. It’s that people don’t want to do it:

Why not extend the X server?

Because for the first time we have a realistic chance of not having to do that.

https://wayland.freedesktop.org/faq.html

I’m not in a position to judge the devs. Maybe the X.Org code really is so bad that you want to run away, screaming in horror. I don’t know.

But all this was a choice. I don’t buy the argument that we never would have gotten rid of things like core fonts.

All the toolkits and programs had to be ported to Wayland. A huge, still unfinished effort. If that was an acceptable thing to do, then it would have been acceptable to make an “X12” that keeps all the good things about X11, remains compatible where feasible, eliminates the problems, and requires some clients to be adjusted. (You could have still made “X11X12” like “XWayland” for actual legacy programs.)