Thankfully you no longer have to be root to start X.

That particular scenario (listening in on other apps) is very theoretical, can you recall any attack ever using it? IIRC Windows still has the same 'ability' - and nobody cares.

The threats models I have seen are easy enough to deal with inside the X ecosystem. And investing all your resources into a new system that is not backward compatible is something no serious security person would do as it leaves your existing users vulnerable