Xorg already provides a full suite of security protocols that allow fine grained control over every aspect of any application down to the single pixmap via access control hooks.

The fact that nobody really uses them should tell you that the sandboxing craze and whitelisting is mostly if not completely security theater and hostile to the general workflow typical for Desktop applications.

Wayland addresses none of those problems. Wayland is merely just a protocol that can be used to blit some bitmaps together. The protocol says nothing about security except that those problems should be dealt with other protocols which are not part of Wayland.