I am not sure what you saw, but on regular Linux processes of the user can spy on each other anyway. In any case, X had the concept of untrusted clients basically forever but nobody cared to invest even the small amount of work necessary to make it work well because nobody thought it would make a different. That this was later used as a major argument against X convinced me that this is not at all about technology.
Yeah, but with how we’re moving towards running each (desktop) application in its own cgroup, thus restricting what syscalls any given application can do, soon any old user process will no longer be able to read any other process’s memory. I don’t believe that the argument about how we need not patch a hole because another one exists right besides it is sound.