I mostly wind up using Arc a lot while using async streams. This tends to occur when emulating a Unix-pipeline-like architecture that also supports concurrency. Basically, "pipelines where we can process up to N items in parallel."

But in this case, the data hiding behind the Arc is almost never mutable. It's typically some shared, read-only information that needs to live until all the concurrent workers are done using it. So this is very easy to reason about: Stick a single chunk of read-only data behind the reference count, and let it get reclaimed when the final worker disappears.