Japan is fairly homogenous with the obvious exception of the Ainu and certain castes. Much more so than Persia/Iran, Russia, Mexico or India. When Japan had a large empire, that was not so much the case, because they ruled over very different peoples.
That's not what most recent archaeological discoveries tell.
Quite distinct groups of humans mixed in ancient Japan, as different and distant at that time as the groups that are mixed in modern times in Peru or India.
Some groups were related to Autronesians, others to Yakuts, yet other groups to Hans, etc.
If I remember correctly, at least three distinct groups are proven to have cohabited and arrived at different times.
I am not talking about prehistory here. The culture of the Japanese "Home Islands" in the Middle Ages consisted of two main groups: Ainu/Edo and the Japonic peoples who spoke related dialects. The first group was treated abysmally and dwindled. There were differences based on environment, e.g. the people in Tohuku had to deal with the cold and those in the south had subtropical climates and more outside interaction. The first major cultural difference (other than Ainu) came in with Christianity in the south and caused a civil war.
Japan is a heck of a lot more homogenous than Taiwan, the Philippines, Indonesia, Malaysia and most of continental south east Asia. In fact, for that matter, Japan was more homogenous in 1707 than the British Isles, Spain, France, Italy or Germany.
The Ainu/mainland distinction is a feature arrived much later than the mixing I am referring to.
My point is that Japan ethnicity is the product of a mixing just as the one occurring nowadays in France, Britain or Norway, between several very different people.
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?
But I don't think we'll reach a common understanding on this topic, so we can just agree to disagree.
"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.
