I don't know if a "frontier mindset" comes from a hetereogenous population in general -- the very phrase "frontier" suggests to me it's more likely to come from

I don't know if a "frontier mindset" comes from a hetereogenous population in general -- the very phrase "frontier" suggests to me it's more likely to come from our history where one kind of people had a "frontier" they were pushing out between them and people they were taking land from by force -- where one society of newcomers was entirely replacing/absorbing another pre-existing society, by force, not just hetereogenous people within a society. Obviously a pretty extreme conflict, that might result in lasting dynamics and mindsets, but the conflict wasn't really the inevitable effect of hetereogeniety alone -- and is of course a very different history than Japan's, and will remain so no matter their contemporary immigration policies.