But assuming threadsafe collections is still a problem - that will never match single-threaded GIL Python running in multiple processes unless some efficient way can be found to optimistically make the collections not threadsafe and only pay the synchronization costs when they're really shared between threads. I think that's a solvable problem, but not with the STM approach PyPy took. Still performance will likely lag the mutli-process solution which doesn't have that cost.
I would rather see a Python that introduces a way to allocate PyObjects in shared memory and with no thread safety. Since it's a new system, that can be done without breaking compatibility. Then sharing between processes could be as easy as sharing between threads and just as efficient. The GIL could stay in that case and the C API could be left unchanged.