Japan's cultural uniqueness is mostly a romantic myth. People act as if the Sakoku period was the norm for Japan and not an exception, but Japan has been just as susceptible to cultural influence as any other country. Much of that "unique culture" was adapted from China, much more of it from the West. That's not in any way calling it illegitimate, a lot of Western countries borrow from one another (or impose cultural change through violence) or crib from the ancient Greeks or Romans. The process of cultural admixture and transformation through trade, religion and violence is the same everywhere. Westerners just seem to ignore the phenomenon as it relates to Japan, I suspect, to maintain some sense of exoticism about the place.

Japan would lose its ethnic homogeneity, but to me that's not something worth caring about or mourning the loss of. The culture would change, but culture always changes.