Code with parallel streams wouldn't even pass my review. The server processes multiple requests simultaneously. It makes no sense to smash all cores in one request. It'll cause bad latency for other requests and will not increase throughput.

There might be use-cases, but I've yet to encounter them.

And when I need parallel computation, I can just use good old ExecutorService. Few more lines, but that's OK for a task that arises once in a 10 years.