Your comment raises a solid use-case I hadn't mentioned: unix running underneath a single-user mobile device. You do need permission systems for that scenario.

Your comment raises a solid use-case I hadn't mentioned: unix running underneath a single-user mobile device. You do need permission systems for that scenario. But they don't have to be in the OS. You could implement them in the application container. A difficulty with security services being tied to the OS is that you're tied to the featureset of the OS, which tends to have a slow-moving API. Much better to implement security via library, so it can move at the same pace as your application platform. There are many ways to build a unix. Unix is strong at hardware interaction (drivers, filesystems), IPC, process control. Security models dilute it. This includes chmod-style filesystems permissions in that. It's lots of complexity that delivers little value. Curious: Stallman built GNU on unix. He's an open guy himself, and had spent a lot of time with MIT systems that didn't implement security in the OS level.