I concur that a human monoculture is unlikely for a long time. Another question that I neglected to mention is whether extraterrestrials would perceive us as having a monoculture by their standards. Well, the problems of humans being unable to recognize aliens ("The Devil in the Dark" and "Home Soil") and of aliens being unable to recognize people (The Motion Picture) have both been treated in Star Trek. Having difficulty communicating with aliens has also been treated ("Darmok"). I think that we shouldn't be surprised if real extraterrestrials end up being far more alien than we're accustomed to seeing in science fiction films and TV. With no examples to point to, I think it's not inconceivable that we might have trouble establishing a common frame of reference, even with aliens who evolved in their habitats similarly to how we evolved in ours. What I'm getting at is that the problem of aliens existing in ways that we can't even imagine might well extend to aliens who evolved also. Even for aliens who we could recognize, and who could recognize us, communication could be practically impossible for cultural or even instinctual reasons. For example, if they were predisposed to regard us something that they must eradicate, it might be impossible to carry on conversations with them of any kind, even though their science might be based on principles similar to ours and even though we could recognize each other.