Suppose thread A (the owner) keeps a reference, but also puts another reference in a global variable. This would increment its local refcount to 1 and have a sh

Suppose thread A (the owner) keeps a reference, but also puts another reference in a global variable. This would increment its local refcount to 1 and have a shared refcount of 1. Then thread B clears the global variable. With your scheme the local refcount would be 1 but the shared refcount would be 0, so thread B would destroy the object even though it's referenced by thread A.