> The thing is a 'day 1' feature everyone hacks back in is clipboard sharing and screen share, each of which remove a decent chunk of these protections. Each essentially grants access to another apps data.

Oh, yes, it's very likely that Wayland will end up with the exact same vulnerabilities. It's very hard in fact to design a windowing system that allows rich interaction between apps, but still maintains per user access control (never mind actually implementing it).