Social Norms, danger or blessing?

Sometime back I watched the anime Full Metal Alchemist, where the concept of equivalent trade was mentioned, i.e. to get something of value, you have to give back something of value. This is an interesting concept, interesting only because why do we have to give something of equal value? Can't it be less or more?

This concept fits in with the social norms of people interaction, where there seems to be a set of invisible rules governing human acceptable behaviour. E.g. when you are late, you would say sorry and try to give a reason (excuse) for being late. Why? It seems not acceptable if you just act 'bo-chap' about it. O.k. so if we accept that there is a general set of human rules for acceptable behaviour, who sets it?

Current day context, it seems to be the 'culture' or 'media' or 'government', setting the trends which formalised into this set of rules, like 'put in effort to get whatever you wants', 'be sensitive'...etc, incl Pop star has to be good looking. Haa, and so when a chinese pop star who is normally represented by a cartoon figure said he is fat, and is going to show his face, someone said she would stop listening to his songs.... strange behaviour, but tacitly acceptable in our society. Strange only becos' you use your ears to listen to songs, not your eyes....

In the past (say more than 100 years ago), the set of rules/norms governs only one society, and we have other distant civilisations as reference points. E.g. some society accords woman with more rights than others, and other societies 'progressively' do so as well when they realised the importance of gender equality. If the entire world becomes normalised against a single set of rules/norms, we no longer has a reference point. So wouldn't there be a danger of collective degeneration? If the entire civilisation becomes a general whole, would we someday degenerates to extremes where we could face extinction? Often in history, we heard of 'lost' civilisations where entire races disappear. Aren't you curious to note that these civilisations typically exists almost in isolation, i.e. they have no effective 'external reference'?

A common set of social norms affects everything else, since it could eventually translate to common languages, cultures, laws ...etc etc. Many of the new chinese and jap languages terms are 'copied' from english, and vice versa. Soon, we would all speak the same language, dress in the same manner, behave accordingly to the same social norms, and perhaps walk together towards collective extinction...