The Unix process, with its uids, is also a form of isolation. But most reasonable people would guess that there are undiscovered privilege escalation bugs in any given kernel and thus be careful who is allowed to put code on a machine.
No. Isolation has always been a security feature, not a reliability feature.
Consider that local privilege escalations are generally (and I'd say correctly) treated as critical severity issues. Any general purpose process memory read-isolation leak is a quick route to a local privilege escalation, so how could isolation be anything other than a security feature? In its absence, you might as well run everything as root and there would be no point whatsoever to caring about privilege escalations.
There was something written lately about how X does not do isolation, at all, easily letting people capture keyboard input that was being typed into a terminal during the sudo prompt.
A screensaver would not be immune to this as well when run in userland, in fact the same session.
I think there should be more isolations in place for cases like this.
[ 0.000000] Kernel/User page tables isolation: enabled
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