For example...say you have 1 2 3 4 and 5.
Say that I can draw a circle around any three consecutive numbers. The one in the middle is a lot like numbers next to it, but each are different. If a draw around 1, 2, and 3, I can call these a "culture". But if I draw around 2, 3, and 4, I can also call this a culture. Say 1,2, and 3 are Britain. And say 3, 4, and 5 is France. While one and two may be an order of magnitude more different than 4 and 5, I'd say 2, 3, and 4 would have similarities that could make the two their own culture like most border-landers. The geopolitical name doesn't really define the culture, it just set's brackets around a section of a cultural continuum.
However, as technology evolves and cultures become more and more connected, similarities become more prominent. This, I believe, is first achieved by the necessity of efficiency and practicality. For instance, a lot of building techniques have been used in the past. This took in factors of material, weather, and social-political concerns. It was also heavily influenced by artistic aesthetic. Aesthetics will never disappear completely, but because of the nature of society as it is now, people will opt for the best ways of doing things more times than not, setting a uniform standard. Another pathway is things like umbrella corporations. When everyone has the same things, and everyone is sharing information about those things, they will begin to use them in the same way. Innovations that would have normally been isolated to a particular place now travel light-speed. The more and more we do things the same way, the more and more alike we will become. Doing things the same breeds a similar outlook of the world. A bunch of carpenters will have a similar outlook on things. A bunch of doctors, dancers, writers, etc. The monoculture will never be total as there never is a total monoculture. But I think, in time, in the same general was that we can call the collective of cultures in America "American culture", a world culture will emerge.
And this doesn't even include political and economic consolidations, but you get the drift. Variety will always be important to the human race, I think, but as the future rolls on, variety will no longer be natural but manufactured. We'd be putting an effort into being different as school kids who go to the same school would, or people at work who wish to be distinguished from everyone else does. Customization will seem like distinct culture, perhaps, but I think it will come down to so many different shades of lipstick on the same pair of lips. Or lips of much, much fewer true varieties than we have now.
Is this bad or good? Don't know. Do we consider the present better than the past because there's far less cultural diversity today than there was in the past. Only times gonna tell.