> We cannot change all user code, so we should change the interpreter code so that it will work correctly with existing user code.

If we made a change that make all user code suddenly incorrect, that change should be reverted. The GC has clear rules about what tp_traverse should and should not do, and we should not violate those rules and make special cases in the gc just because we forced some classes to be incorrect. This will make much more difficult to reason about GC bugs, the tp_traverse implementation of classes and much difficult to maintain the GC itself.