What level of "isolation" forms that threshold though? We're nowhere near a single worldwide religion, for example, which is odd because it's the most existential of cultural beliefs.

We have neoliberalism, at least

When thinking about this cynically, I feel like this has some depressing ramifications. Cultural differences (and isolation) are often born from financial disparities as well. Does this mean that the path to global cultural homogeneity is basically just gentrification on a massive scale where economic -> cultural minorities get more and more ostracized?

Toshi would tear down his local shrine and import the third world because it's better than having some Judeo-Christian reporters call them names?

The cultural and ethnic shifts happening are happening because banks, governments, and CEOs see it as being in their own interest. More working hands, more mouths to feed, more votes for their party. None of it has ever been natural and none of it is to our benefit

He would have if those foreigners were similar in numbers to the Japanese, rather than being a minute minority.

The result depends on the ratio. I don't see why we can't have trade and relations with other countries without also assimilating eachother

I think what makes it different this time is that we are using science to find the best culture. "How should we behave towards each other for the best possible outcome?" If a certain culture is clearly and demonstrably better, then that will lead to everyone adopting it.

Democracy might also be part of it, as in authoritarian states the person in power wield such a an outsized influence that their personal whims and fancies can turn into culture.

For example, most people in the world, nowadays, send their kids to school and use smartphones. And even though Putin, Xi and Western leaders may not be friends, they wear basically the same professional attire.

200 years ago, there would be much fewer common cultural traits.

Homogenization doesn't seem to happen until either some sweeping administrative body insists on it, or (unique to modernity) a specific culture's media successfully and pervasively inserts itself between people and local/inherited traditions.

So, when there's more cultural exchange and less isolation?

Most people didn't leave their local area, the need for a unified language was extremely limited compared to modern society. Dialects developed because interaction was so low, so you could easily hear which part everyone came from, today all of that is evening out over time. Go a few hundred miles and people use different words even if it is the same language and people would have a hard time understanding each other.