every culture in history, in every time and every place, has operated from the assumption that it had it 95% correct and that the other 5% would arrive in five years’ time!

Don't believe it.

Cultures, to the best of my knowledge, differ somewhat significantly with respect to their attitude to moral and ideological progress or decline. It doesn't even seem particularly likely that every culture in history has even had an attitude such that it can be said to be operation with an assumption one way or the other.