My guess would be that a lot of what makes Tokyo work is its high-trust society, rather than its (homogeneous) demographics per se.
Building a society with high trust seems historically path dependent, so a lot of the things that work well in Tokyo might not work well in the US.
For example, if you can trust that new restaurants will not cut corners on cost, health inspection becomes more of a formality. If you can trust the building will be left in good condition by businesses, it is more acceptable to rent a place out to whoever wants it, and so you see as time goes by that a small cornerstore becomes an upscale hairdresser, which becomes a laundromat, which becomes a take-out restaurant.
Homogeneous demographics might help, but I suspect building a high trust society is more culture than demographics. For example, a religious group may create a high-trust enclave by promising to excommunicate members who violate norms, while a demographically homogeneous country may destroy its culture in civil war and build something low-trust, too (China).
> building a high trust society is more culture than demographics
This is the kind of question that benefits from a probabilistic analysis: does ethnic homogeneity, on average, help or hinder building and maintaining a high trust society?
Many conflicts in world history have an ethnic dimension. Many substantial social cleavages, even if they are not ethnically based, over time may evolve an ethnic dimension. For example some religions have an ethnic dimension, most religions require members to marry only within the own religion etc. Moreover, people are more likely to procreate with people they spend a lot of time with. So even linguistic differences can lead to ethnicisation, since it's easier to socialise with people you can easily communicate with.
For example, if you can trust that new restaurants will not cut corners on cost, health inspection becomes more of a formality. If you can trust the building will be left in good condition by businesses, it is more acceptable to rent a place out to whoever wants it, and so you see as time goes by that a small cornerstore becomes an upscale hairdresser, which becomes a laundromat, which becomes a take-out restaurant.
Homogeneous demographics might help, but I suspect building a high trust society is more culture than demographics. For example, a religious group may create a high-trust enclave by promising to excommunicate members who violate norms, while a demographically homogeneous country may destroy its culture in civil war and build something low-trust, too (China).