Agreed that the idea of a monolithic Western culture is a farce, but the idea that Western culture with all its variations was different in many respects from other cultures is not. I maintain there are vast differences between and within cultures. Further some of the key lasting impacts of the colonial and imperial eras has been in putting different cultures into contact with one another, mixing cultures together, and changing their evolution. Differences still persist however.
You seem unaware. You write like someone who has never spent time outside the US. Please take a trip to Japan, China, Vietnam, the Philippines, Thailand, Malaysia and Indonesia. Then go to India. Then onto the Mideast and North Africa. Then south to Nigeria,Ethiopoa, Rwanda, Kenya and more. Once you get past the tourist and Western enclaves, you may be struck by how distinctive the cultures are.
The culture of the US is a distinctive mix of Native American, Western, Black African, Mid-East, and Asian varieties. Melting pot is too strong, but there is still a dominant process of mixing and homogenization occurring within the US and between other nations. Against this stands the threat of disintegration from importing too much diversity from immigrants who reject integration into the dominant US culture. Perhaps you could adopt a more balanced view on what differences we wish to preserve and discuss how to integrate others into our combined culture.