Honestly, this is why I find "systems language" kind of an annoying term, because you're not wrong, but it's also true that we're talking about two different things. I just don't think we have good language terminology for the different sorts of languages here.
> could you write Erlang in Erlang entirely
I think this sort of question is where theory and practice diverge: sure, due to turing completeness. But theory in this sense doesn't care about things like runtime performance, or maintainability.
> But personally I don't care about that enough
Some people and some domains do need to care about implementing the low-level details of a system. The VMs and runtimes and operating systems. And that's what I meant by my original post.
So, as the author of not one but two operating systems (one of which I've recently published, another will likely never see daylight): I've never felt the need for 'async/await' at the OS kernel level. And above that it is essentially all applications, and there almost everything has a runtime, usually in the form of a standard library.
I agree with you that writing Erlang in Erlang today is not feasible for the runtime performance matter, less so for maintainability (which I've found to be excellent for anything I ever did in Erlang, probably better than any other language I've used).
And effectively it is maintainability that we are talking about here because that is where this particular pattern makes life considerably harder. It is hard enough to reason about async code 20 minutes after you wrote it, much harder still if you have to get into a code base that you did not write or if you have to dig in six months (or a decade) later to solve some problem.
I get your gripe about the term systems language, but we can just delineate it in a descriptive way so we are not constrained by terminology that ill fits the various uses cases. Low level language or runtime-free language would be fine as well (the 'no true Scotsmen of systems languages ;) ).
But in the end this is about the actor model, not about Erlang per se, that is just one particular example and I don't see any reason why the actor model could not be a first class citizen in a systems oriented language, you could choose to use it or not and if you did that would have certain consequences just like using async/await have all kinds of consequences, and most likely when writing low level OS code you would not be using that anyway.
I mean, I'm also not saying async/await is critical for kernels. I'm only saying that "everything is an actor" isn't really possible at the language level.
Async/await is used for a lot of RTOS like things in Rust. At Oxide, we deliberately did not do that, and did something much closer to actors, actually. Both patterns are absolutely viable, for sure. But as patterns, and not as language primitives, at least on the actor side.
> could you write Erlang in Erlang entirely
I think this sort of question is where theory and practice diverge: sure, due to turing completeness. But theory in this sense doesn't care about things like runtime performance, or maintainability.
> But personally I don't care about that enough
Some people and some domains do need to care about implementing the low-level details of a system. The VMs and runtimes and operating systems. And that's what I meant by my original post.