> I’m speaking of the consequences of Xorg’s basic lack of isolation (because it wasn’t designed for use cases where that matters): that granting access to the X socket allows you to inspect, intercept and modify inputs and outputs for things like key strokes, window contents (yours or others’) and clipboards.

This is not true! Both the protocol and the implementation allow for significant separation of server permissions (for example, notice the difference between ssh -X and ssh -Y), and the peer-to-peer nature of things like clipboard means it is quite easy to deny requests on an application level if they're written for it.