Both of your statements are incorrect. Locks are an abstract concept that have many different behaviors. In a multi-threaded world (not JS) a certain type of synchronization lock, such as a single-writer/multiple-reader locking mechanism would specifically allow you to unblock all waiting coroutines. Another type of lock might only allow the first waiter to unblock.

There simply is no concurrency in JS though, so there is no need for locking. There is no way that you would ever "unblock all coroutines waiting for the promise" because you just can't run more than 1 coroutine at a time. So your problem is with ordering, not concurrency and locks are a solution for concurrency.

And the simplest state machine is a boolean variable.

Anyway, I guess if you still disagree then you can go and ask the people who build JS engines why they don't want to add locks. Maybe ask the v8 team - they add stuff that isn't in the spec all the time and they haven't seen a big need for this, so they must know the answer...