I fondly remember Joe Armstrong, who was a good friend.

However, not having a mathematical foundation has been a

severe limitation for Erlang.

The Erlang community is trying to dig its way out with

modifications and extensions. Still have a long way to go.

Of course, no Actors framework is perfect.

Many important implementations have been done Erlang.

Also, significant Erlang projects are ongoing to good effect.

not really, smalltalk's "actors"[0]/objects are organized around data encapsulation principles. Erlang's "actors"[0]/processes are organized around failure domain principles. It's not unreasonable to go in one direction (making your data encapsulation match failure domains) but if you try to organize Erlang's "actors" around data encapsulation you will wind up with very shitty code (hard to read, hard to test, hard to debug, unnecessary performance regressions), unsurprisingly, because that is not what they were meant to be.

[0] I put actors in quotes because neither are true actor system, either. Nor were either designed to be, though if you squint they look similar.

See the following for a rigorous definition of the Actors Abstraction:

https://papers.ssrn.com/abstract=3418003