Might be a nitpicking here, but GenServers aren't useful for concurrency. They just manage state, and only process one message at a time. If you're using this as a cache, your reads will be bottlenecked by however quickly the GenServer can handle the read requests.
I can dump 1000s of things into its message queue and then do something else. It'll keep working away.
It's like saying threads aren't useful for concurrency because they can only do one thing at a time.
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.