I'm not criticizing other cultures nor idealizing the mainstream one. I'm just pointing out that large efficiency gains can be had from having fewer and bigger cultures.
However, this optimization makes also our civilization more fragile, so the tradeoffs are more complex than in your version of the theory.
That was the fundamental belief behind the eurocentric approach to anthropology, and other ethnocentric supremacist belief systems which from ignorance brought a lot of hardship to the world.
You don't have to look far enough to see how your culture, not long ago, was just as niche and pointless as the ones you're criticising now. You should also know that dominating cultures have already succumbed into irrelevance in spite of all the supremacist beliefs. Perhaps there's something to be learned about poopooing other cultures