Depends on the level of abstraction you’re looking at. Operating systems today are vastly more capable of organizing different provesses, distributing work amongst multiple CPU cores, CPU caches, etc. I guess the von Neumann architecture has just proven really successful in practice. And von Neumann machines require a certain set of capabilities in their OSes.

Maybe look at embedded systems, where we find a bit more variety. Things like DSPs or microcontrollers.