This is why I think multitenancy is the more important problem (though both are related), because it's the key to solving shared-kernel application permissions

This is why I think multitenancy is the more important problem (though both are related), because it's the key to solving shared-kernel application permissions without "users". Containers were a step in the right direction but aren't a sufficient security boundary in themselves - what is currently handled by the "container runtime"/sandbox needs to be built into the kernel IMO.