That only works in some cases, if the boundary between Python and native code is absolute. In many cases users want to extend/configure the behavior of that nat

That only works in some cases, if the boundary between Python and native code is absolute. In many cases users want to extend/configure the behavior of that native code, e.g. through callbacks or subclassing, and the GIL makes the behavior prohibitively slow (needing to lock/unlock to serialize at any of these potential Python<->native boundaries) or unsafe (deadlocks/corruption if the GIL isn't handled). There's a lot of C++ code bound in python (e.g. via pybind11) where the GIL currently imposes a hard bound on how users can employ parallelism, even in "nominally" native code.