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So, if you want this inflexible model, you could easily build it as a library in Erlang, just as you're clearly using someone's inflexible fork-join model library (or an inflexible language implementation); in the mean time you're missing out on things like fork A, B, C, A expected to run much longer than the other two, when B and C return, fork D using the results from B and C , finally wait for A and D to return. Or fork A, B, C and merge the first two that return, then merge that one and the third. Or fork these ten jobs, but only run four of them at any given time (resource constraints). My first example you could probably structure into your model with an extra fork; the second example won't fit in your model, the third fits if you fork four workers and add a shared queuing mechanism, but that feels more complex.

These kind of techniques are key to using parallelism to reduce latency. Always having to wait for everyone to finish at each step makes for a lot of waiting.