human social mores seem to change on the order of 20-40 years which is consistent with the amount of time it takes a new generation of people to take the helm and for the old generation to die out

If there's a causal link here, then it's possible the biggest problem with social change and technological advances would be due to increased longevity, in which case it might not matter how long the time span is... even if there were decades, it wouldn't be enough.