There are two ways a language can help 1. A performant "i/o" layer in the standard library that allows a large number of concurrent activity (forget thread vs

There are two ways a language can help 1. A performant "i/o" layer in the standard library that allows a large number of concurrent activity (forget thread vs coroutine differences). 2. Ability of programmer to express concurrency. Ideally, this has nothing to do with "I/O". If I am doing two calculations and both can run simultaneously, I should be able to represent that. Similarly for wait/join. Explicitly marking a thread as async-only will just force everyone else (who need sync and cannot track/return a promise/callback to their caller) write a wrapper around it for no reason.