> Shell is great for many common tasks, but I would never call it beginner-friendly, even less "trivial to use for concurrency".

Yet somehow beginners in my lab cannot get their hands off the shell and we have to politely guide them to other languages... But still, I insist that parallelism in the shell is very easy. How do you do that in Python, for example?

    # convert images from jpg to png, in parallel
    for i in *.jpg; do
            echo convert $i ${i/jpg/png}
    done | parallel

Regarding your "complicated" use cases, I must confess that I don't understand them nor their relevance. But for simple people like me without fancy needs, the simple concurrency offered by GNU parallel and GNU make is good enough.