The cracks are already there though in Japan where this is starting to fall apart and has fueled a substantial rise in anti-immigration sentiment there. Particularly at specific groups that are not assimilating (but affecting basically every immigrant).

I feel like half your post is dancing around not saying this.

This is the flip side of East Asian societies where everything is polite and orderly: they're also extremely conformist (sometimes stifling so), and the conformity and good manners are maintained by strict social policing (it's not much fun to stand out and be different)

I'm not saying this, because I'm not aware of this. I have a friend who is enjoying living in Osaka, but I wouldn't feel qualified to talk about Japanese sentiment towards him or any other group of non-natives.

I'm saying that culture spreads through fear of being ostracized and the cost of not conforming to that culture. This is true in any country. It seems Japan has a "broader" and "firmer" culture that is rooted in respect to others, but it would take some serious effort for a similar culture to take hold in Europe or the US, I think.

A sense of "non conformity" leading to issues is not unique to Japan though, in Europe it is described as a problem with "multiculturalism".

The British (of which I am one), have spent thousands of years stealing the best bits of immigrant culture but seem to have forgotten that (or have been unable to absorb, for whatever reason), for the last ~60 years. That's leading to a rise in far-right sentiment and a growing desire for mono-culturalism. I can see evidence of that in Germany, France and other European nations. I could easily imagine it appearing more in Japan.