> No shared mutability. This is an unrealistic constraint;

Hard disagree.

It is impossible to safely use shared mutable state b/w parallel processes.

See how DB isolation levels work - serializable is the only safe way to go if your processes do anything conditional on the current state. To parallelize, you have to partition the DB which is kind of a cheap way to split one physical DB into separate logical DBs.

So even DBs - the largest shared mutable states we have - are also not really shared mutable states. They need to be broken down into unshared mutable states to be useful.

Shared mutable state is a smell both at the code level and system level.