This is some bullshit. And it doesn't help that everyone is using async to mean different things.

Blocking/synchronous system calls are not friendly to userspace async runtimes. Explicit async runtimes are a pain in the ass. The best solution imo is a blocking userspace interface that maps into the system however it likes. Go and Haskell do this. Erlang and probably all BEAM languages do too. Nim and Zig let you choose.

Runtimes like Go and Haskell can map onto code that makes blocking system calls for compatibility reasons when needed, through explicit request to the runtime, or detecting that a green thread has blocked.