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Criticize the following idea at will: let's see if there's a nugget of truth in it or if it busts under its own weight.
Moral progress can be modeled as the strengthening of a society memetic immune system.
Two extreme cases to illustrate this: a society where the power is held by an elite with very strict and very uniform moral code. In that case, any stray meme will likely be in conflict with the elite's memeplex, and so it poses a threat to the society itself. The immune system is very weak, the society is very oppressive. Another case: a society where power is distributed or the elite derives its command from other means than a very tight set of ideas: in this case, not many memes will undermine society stability, its immune system is much stronger, the society is more tolerant and progressive.
I think you're only looking at one failure mode-- the excessively brittle society. Living organisms have boundaries that let some things in and refuse others.
A society needs to support some ideas, but not all ideas. For example, your society with the strong immune system needs some way to stabilize itself against going authoritarian.