It breaks everything of the sort. On X11, a client can view the contents of any arbitrary window, simulate input, read input — this is necessary obviously for

It breaks everything of the sort. On X11, a client can view the contents of any arbitrary window, simulate input, read input — this is necessary obviously for screen sharing applications; though one can argue that maybe a permission model needs to exist to allow some clients to run with lower privileges — the funny thing is that it does; it's called te XSECURITY extension, it's very easy to use, and seldom used for whatever reason though SSH uses it whenever it can. On Wayland, no client can do such things, but many clients such as screen sharing tools, profiling tools, and whatever else that needs to simulate input and read surfaces obviously needs to. Wayland developers simply say “It must be built into the compositor”, but no compositor can be expected to provide all that, and no compositor does.