Where is the major win? Sorry but I just don't see the use case for free-threading.

Native code can already be multi-threaded so if you are using Python to drive parallelized native code, there's no win there. If your Python code is the bottleneck, well then you could have subinterpreters with shared buffers and locks. If you really need to have shared objects, do you actually need to mutate them from multiple interpreters? If not, what about exploring language support for frozen objects or proxies?

The only thing that free threading gives you is concurrent mutations to Python objects, which is like, whatever. In all my years of writing Python I have never once found myself thinking "I wish I could mutate the same object from two different threads".