It is expensive to honor ancestors, and ancestors don't demand that we continue to maintain that value, so it receives less attention.

That's something different - a human trait that makes us want to avoid expensive commitments while paying them lip service. A self consistent system would not have this trait, and would keep "honor ancestors" in it, and do so or not depending on the cost and the interaction with other moral values.

If you want to look at even self-consistent systems being unstable, I suggest looking at social situations, where other entities reward value-change. Or a no-free-lunch result of the type "This powerful being will not trade with agents having value V."