Synchronous systems can benefit from backpressure. Clients can and do open multiple concurrent connections to such systems (common to work around the inevitable

Synchronous systems can benefit from backpressure. Clients can and do open multiple concurrent connections to such systems (common to work around the inevitable throughput limitations of synchronous systems)... it is still helpful to tell any given connections, no, I'm at my concurrency limit, any work you give me is just going to sit in a queue and increase your service time, so please don't.