[dataflow]

> There's no good reason (that I can think of) why this shouldn't have been the case all along

Determinism can decrease performance dramatically. Like concatenating items (say, object files into a library) in order is clearly more expensive in both time & space than processing them out of order. One requires you to store everything in memory and then sort them before you start doing any work, whereas the other one lets you do your work in a streaming fashion. Enforcing determinism can turn an O(1)-space/O(n)-time algorithm into an O(n)-space/O(n log n)-time one, increasing latency and decreasing throughput. You wouldn't take a performance hit like that without a good reason to justify it.