That does seem like a lot to give up however if doing any amount of I/O. No?

If "any amount" means millions of concurrent connections maybe. But in reality we've build thread based concurrency, event loops and state machines for decades before automatic state machine creation from async code came along.

Async doesn't have access to anything that sync rust doesn't have access to, it just provides syntactic sugar for an opinionated way of working with it.

On the contrary, async is very incompatible with MMAP for example, because a page fault can pause the thread but will block the entire executor or executor thread.

I'd even argue that once you hit that scale you want more control than async offers, so it's only good at that middle ground where you have a lot of stuff, but you also don't really care enough to architect the thing properly.

Thanks for the perspective. Very much appreciated.