As a long-time C# dev, now full-time Erlang/Elixir dev, I've written things like this before myself when attempting to bring some of the FP tools I loved back to C# for my own benefit - so this looks really familiar in that sense. This is by no means an actual competitor to Erlang's actors - for one thing it lacks any form of supervision, which is really the most important aspect of Erlang's actor model (i.e. the "let it crash" philosophy). There are other things missing as well (such as the OTP behaviours, e.g. gen_server, provided by Erlang).

That said, this looks like something I'd definitely use, or at least experiment with in C# if I was still doing work in the language - though for the actor bits I would probably lean on Orleans.