The interesting thing about the second-system/third-system distinction is that it gives a strong impression that no one really knew exactly what an operating system should be until various examples were built. Arguably, that leaves open the possibility that further forms of OSes exist but aren't easily found due to the difficulty of putting together a system.
So many layers are now built on top of the OS that it seems the most obvious way it will continue to grow is by gradually subsiding abstraction layers above that are so common, you'd never not want them.
This has already happened with things like sound drivers, TCP/IP stacks, firewalling, etc. but will continue upward
