I don't think it is a universal solution. What if I am interested in parallelism as well, only not for the coroutines that operate on the same data? If my app handles 10k connections, I want them to be handled in parallel, as they do not share anything so making them parallel is easy. What is not easy is running stuff concurrently on shared data - that requires some form of synchronization and async/await with event loops is a very elegant solution.
You say that it can be handled with an SPSC queue and it is only one ack/rel. But then add another type of event that can happen concurrently, e.g. a user request to reconfigure the app. Or an inactivity timeout. I can trivially handle those with adding more branches to the `select!`, and my code still stays easy to follow. With threads dedicated to each type of concurrent action and trying to update state of the app directly I imagine this can get hairy pretty quickly.
In any case acq/rel is the only thing required here and it is extremely cheap.
edit: in any case we are discussing synchronization and 'has_free_space' 'had_data' are a form of synchronization, we all agree that async and threads have different performance characteristics.