Currently Python only allows certain thread interleavings, because those are the points where the GIL is released. If you no longer had the GIL, the set of possible thread interleavings would increase. If your application is not safe with a particular interleaving, it can now break. That's a breaking change, so would require Python 4.

It might be possible to retain current interleaving semantics, through research-level techniques such as transactional memory, but these do not seem likely anytime soon despite a lot of work.

It's also unclear how tractable it is to retain current garbage collection semantics without a GIL, so that could have to change as well with similar issues and similar possible solutions which seem unlikely to land anytime soon.