If you want easier concurrency with higher overhead, that's exactly what OS threads do. The OS kernel is the "event loop" in this case, and they're highly optimized for this, though there is some overhead which is hard to avoid.

Rust has gone back and forth on this. Pre-1.0 versions of Rust had a "green threads" mode. It was removed in favor of OS threads because it wasn't worth the complexity.

Go is a language in which "all functions are async" and it's pretty cool. But it needs a runtime, and interop with C / C++ has some real overhead related to switching to a C-style stack. Rust did not want to make those compromises, it wanted to be appropriate for bare-metal or kernel programming, and for libraries used by big high-performance C++ applications. In these cases, Go's model does not work well.

I personally use Go a lot, it's great for network services and command-line tools, and that's mostly what I do.