Many projects have solved this problem with dual compilation modes and provide two binaries the user can select from at runtime. Eliminating the GIL doesn't ha

Many projects have solved this problem with dual compilation modes and provide two binaries the user can select from at runtime. Eliminating the GIL doesn't have to mean actually eliminating it. You could certainly have #defines and/or alternate implementations that make the fine-grained locks no-ops when compiling in GIL mode. Conversely make the GIL a no-op in multithreaded mode.