I think it was the "security" argument. Some magic words like "security" and "protect the children" make most people go insane, abandon all reason.
Xorg's security (and X11 in general) is objectively terrible - this is not even a debatable question.
Are you seriously acting like every application having full access over other applications is not an actual security issue?
You'll excuse me if I am not terribly enthusiastic about breaking all sorts of functionality to solve a security problem I've never experienced in my 25 years on linux.