[_qbjt]

> GenServers aren't useful for concurrency.

Sure, maybe not by themselves but typically you would run many GenServers concurrently in your application as part of a supervision tree. Libraries like Broadway (and the underlying GenStage) are essentially just leveraging GenServer to make it easier to orchestrate concurrency and state synchronization across multiple processes in your application. But you could build a comparable system on your own just using GenServer and a dynamic supervisor.