The bank has the best doors, the best locks, and the best cameras, and it is patrolled by a guard who props the doors open to so he doesn't have to keep fooling with the locks and points the cameras the other way to extend his smoke break. SeL4 would be another system used by humans.

It's always possible to break a perfect system by moving an additional layer of abstraction outward, and attacking one of the assumptions upon which it's built. Some of our era's highest security systems - game consoles - have been broken by undervolting them until the logic failed.

I once worked on AUD 450M banking project, the root password was kept in a kickstart file and unchanged, root SSH was allowed. The bank didn't care until I told the external security auditor who included it as part of their report.