In the company I'm working for, we had to spent more engineer time on GIL workarounds (dealing with the extra complexity caused by multiprocessing, e.g. patching C++ libraries to put all their state into shared memory) than we needed for the Python 2 -> 3 migration. And we've only managed to parallelize less than half of our workload so far.

Even if this will be a major breaking change to Python, it'll be worth it for us.