A viable solution for Python concurrency

I think that just means that running present-day extension code requires the same "stop the world" operation that GC does in this design, so it would be possible to provide the same behavior as today, at the cost of having C extension parallelism stay as bad as Python parallelism is currently. Of course, you'd want to allow a C method to declare that it's okay if Python code can run between its library calls, so that you can make extensions that do parallelize well.