> "broken by design" is an extraordinary claim and requires extraordinary evidences

There are many technical aspects that make Wayland broken by design (like default forced vsync, forced double buffering, a fucked up event-loop for single threaded applications or severely lacking functionality for things like window positioning or screen sharing). But the biggest problem is the design-philosophy: Wayland makes life extremely easy for gate keeping "Protocol Designers" and extremely hard for application developers.

> un-sandboxable

Not true. The quick and dirty way would be using Xephyr. Besides that many access control hooks like XACE are present and standardized in the X11 protocol for many years. Application developers just choose not to use them. So if X11 is not secure enough for you, blame GNOME and KDE, not X11.