See also Sun's project barcelona from a decade+ ago: http://www.oracle.com/technetwork/articles/javase/mvm-141094...

Very cool. I was still writing assembly at college at the time.

Define "operating system"? It's not crazy to think that you might have another layer inside your VM, not unlike how a hypervisor was one layer that didn't exist outside of mainframes until adapted for regular computers.

In a world of unikernels and library OSes, that is the job of the runtime.

Not necessarily. One of the ideas behind unikernels is very fast boot time, so that you can start a new instance for every tenant. Another idea is reduced complexity...

In which the OS and language runtime are the same thing, e.g. Mirage OS.