alt Hacker NewsPreservation not in the stricter sense, but meant as "reasonable slowly moving culture" as it implies stability. You also need coherence to some degree to maintain a functional society. As an hypothetical extreme example imagine one half of the population not even understanding the language of the other half, such a society would break apart real quick.
Your assertion on coherence for a functional society and your extreme hypothetical is fallacious. Multiple governments and societies across the globe operate effectively without a single national language.
The united states for example, current situation acknowledged, saw a meteoric rise even with, perhaps even because of a rich cultural tapestry that could be in no way described as coherent. Sure there are large blocs of cultural identity across multiple regions, but that's about it, and even then that's been on the decline for some time.