"So that, if such mixing produces great results (do we agree that modern day Japan is that?), why not welcome today's mixings for the sake of the great nations of the future?"
It can also produce indifferent and awful results depending on the situation, which you don't mention.
Do you think that the introduction of the Italian Mafia was beneficial to the USA for example? It is clear that most Italian Americans were never in the Mafia but the USA let in individuals who were known to be in it, which was avoidable. In fact, the USA revitalised it after WW2 in Italy too.
Japan's remote ancestry isn't the issue here. Your point is not well nuanced and merely promotes the dominant narrative of today. If Japan has been great, I would put that down to a combination of native resilience (since Japan can be a tough environment in terms of earthquakes, tsunami and volcanos), Chinese high culture (especially in art, philosophy and literature) and a high degree of adaptability to western technologies. Austronesians may have contributed some level of maritime knowledge, but that would have been superseded at least a thousand years ago by improved ships and navigation methods.
Japan is at a crossroads just now. It does appear to have some economic and structural issues. Where these go long term, we will see.
The Ainu are far more relevant than what you talk about, because they still exist today and are under major threat. Their future, unfortunately, is not well assured.