That's not the implicitness I'm talking about. While it sounds like loading old C modules will just re-enable the GIL, the problem is that they will never be updated to not rely on the old Python concurrency model. All that C code was written implicitly assuming that certain blocks of code were surrounded by a GIL.

It could be a real headache for any hoped-for transition to nogil Python if lots of GIL-reliant C code is floating around where there's little hope of updating it without having to worry about subtle bugs popping up. And even if the conversion was risk-free (which I doubt), many organizations will still not want to dig into their legacy C codebases and make significant changes.

And the probability of a given piece of C code that was written for a thread safe environment working when called from multiple threads at once is pretty low for anything not 100% purely functional.