It wouldn't be doomed. Foreigners could have their cultural enclaves and Japanese theirs. Anyway, this is an extremely minor concern relative to the astronomic benefits of open borders. See my comment here: https://news.ycombinator.com/item?id=9769632
Your comment is very thought provoking but I don't agree the culture wouldn't be ruined nor do I agree it's a minor concern. It's already visibly being ruined by the massive influx of rude Chinese tourists.
Cultural enclaves don't say anything about crime like the fact that 19 times out of 20 if you lose your wallet in Japan you'll get it back with all the money in it. It's hard to imagine that would stay true with a much larger percent of non Japanese culture people.
The children of immigrants who grow up in the new culture learn it quickly and it becomes their own. American Chinese who grow up here behave just like white Americans. And BTW the Japanese thing you mentioned is likely almost entirely due to more efficient & convenient lost and found process: https://www.psychologytoday.com/blog/culture-conscious/20140...
