The big constraint (aside from backwards compatibility) is performance: Guido has indicated that he is unwilling to accept much (if any) slowdown of single-threaded code in order to remove the GIL. It's (relatively) easy to remove the GIL and replace it with a bunch of fine-grained locks (or atomic increments, etc), but doing so tends to slow things down. The challenge is in figuring out how to avoid synchronization overhead for common operations (mainly reference counts).

It's true that buffered refcounting probably means that `__del__` would no longer be called immediately as it is now, but I'm not sure if that's a requirement - pypy and jython don't do this either, and destructors are generally discouraged in favor of `with` blocks these days.