We're actually creating new unique and diverse cultures by adopting and remixing parts of the various cultures we encounter. Just looking at photos from other cultures (as you have done today) is a part of that process. This is a good thing. Some of my favorite things are different takes on a thing that came from another culture. Diversity really does make us stronger and it makes our lives richer. While it might be neat to see what results from cultures being totally isolated, I think it'd be much more interesting to see what results from bringing those cultures together.
The truth is that none of those "unique and diverse cultures" you're mourning the loss of came from anything different. Even japan's culture, although it was one of the more isolationist nations, was still massively influenced by other people/places. Technology accelerates the process, but the process itself is unchanged. It made 19th century japan what it was then, just like it made japan what it is now. It's just what humans do and always have done. There's no reason to feel sad about it.