Summary:

- We don't need separate processes if the language we use is inherently safe (e.g., modern Common Lisp).

- Without process isolation, it is possible to share large, complicated and possibly mutable data structures (graphs, arrays, user-defined objects) system-wide.

- Once it is possible to cheaply share and communicate arbitrary data structures, it is pointless to maintain designated 'file system'.

- Not having a file system raises the question of what data should be persistent and what shouldn't. But on a modern computer, it is feasible to just treat all data as persistent (maybe excluding the youngest GC generation).

The best news is that the author is actually working hard to implement this operating system. The first part - the Lisp implementation - is already in a pretty good shape and could be finished within the next two years: https://github.com/robert-strandh/SICL