The resource management doesn't fit an arena model, that is an overly simplistic view of what is required. The resources have a lifecycle independent of the references any execution context has with them. A reference isn't ownership, it just keeps the resource from disappearing or pins it in a particular context.

Memory allocations may not actually exist in memory. They can be paged to disk in user space because, frankly, the virtual memory hardware on modern CPUs is often incapable of handling large storage and when it does the performance is poor. Sure, you can do better than the default allocators in Rust and elsewhere but that is far from the only reason you might want a reference counter on a resource. DMA requires managing reference counts for threads of execution that only exist in silicon, not in software.

Arenas solve none of this. The lifecycle of the resource has nothing to do with the lifecycle of the execution context of a coroutine.