If your tests are ensuring buggy behavior, then... you're writing your tests wrong?
What are you even trying to say with this? I've never once seen a unit test intended to enforce a bug...?
But you cannot allow it to change because your users are relying on the existing behavior.
And you're using that... to justify that the "only" thing unit tests achieve is maintain bugs? Although in any case, this is no longer a bug, it's part of your spec now.
But regardless, what about the other 99%+ of unit tests that are enforcing correct behavior...?
Your argument is like saying that food is bad because people occasionally get food poisoning.