I think it's a hypothetical, not an oxymoron. A monoculture could exist, but if it did, you'd just call it "culture," which maybe is what you're getting at: The

I think it's a hypothetical, not an oxymoron. A monoculture could exist, but if it did, you'd just call it "culture," which maybe is what you're getting at: The moment it is realized, the qualifier is redundant. I don't know if an English word for such a thing exists.