It actually is feasible to sandbox X11 in that way, I believe this is what security-focused distributions like Qubes and Whonix do. But yeah, it is essentially

It actually is feasible to sandbox X11 in that way, I believe this is what security-focused distributions like Qubes and Whonix do. But yeah, it is essentially working around the problems with X11 and not really suitable for ordinary use. To me the real progress needed is that applications need to move to using sandbox-friendly APIs so they don't break when put in the sandbox, and moving to Wayland is a key part of that. Applications built around the X11 idea of always having full access to everything will need changes regardless of what sandboxing solution is in use.