I've worked on Japanese dev teams, and I've always liked the Japanese geeks I've worked with. They're usually out of the running for "serious" jobs at the company, so they tend to be much more relaxed and fun than the "important" people.

But I'm not at all convinced that most of them could make much more money outside Japan. I've almost always had to communicate with Japanese devs in Japanese, because few of them spoke much English. They tended to be kids who didn't do all that well in school, including English class, so they didn't go to prestigious universities, whose students tend to go into more respected professions.

Things might have changed since I lived there, but Japanese devs were not valued very highly in Japan and most would have a hard time being more valuable outside Japan.

The outside world is not Japan. That is more than enough to keep most from wanting to leave.