If your idea of the health of Japan is totally disconnected from whether the Japanese -- technically -- exist, then no, we don't.

When the English and the Germans came to North America and replaced the population there, was that a success for North America? If so, is it the kind of success you'd like to have someone else give to you?

Over 10s of generations, I'm not especially worried about the trivial differences in the genetic ancestry of the people who live in a given country.

Most people don't wish to die for their ideology. You may be an exception. I doubt it, but if so, you're welcome to it.