It's a planet. We are a species. Fragmentation only occurs due to focusing on our perceived differences and conforming to our identified cultural stereotype. When we focus instead on a our similarities and common goals we are capable of being both diverse and unified. This doesn't have to mean giving up your culture, it just means accepting that despite other people having theirs, we can still cooperate. Without cultural exchange, we'd be centuries behind in science, art, philosophy, and pretty much any human endeavor. Passive segregation limits our collective potential. Nationalism more often leads to more conflict than collaboration, and attempts to homogenise a culture. Even within a nation's population, there is a diverse range of subcultures, and many individuals who break the stereotype of the group they identify with. Apparently some women are actually engineers. Some Chinese people aren't even that good at maths. Some old white guys care about poor people and the environment. Some Aussies don't give a shit about AFL or sport in general. Some black people are terrible dancers. Perceived racism is a problem. It may be passive or unintentional, but it has an effect and it is not positive. It reinforces the fragmentation you fear. Multiculturalism allows us to enjoy a choice of kebabs, sushi, curry, pizza or a burger. Australia is built on the back of migrants, and not all are western European. IMO, it's a major part of our culture.