> With an increase in immigration and a cheapening of robotic workers and kiosks... where is this decrease in unemployment and increase in labor-force participation going to come from?

I don't know where it would come from (perhaps the same magical unicorn tooth fairy who would remove balkanization, inspire durable construction, and give us more passenger rail), but the US would look a lot stronger if it had it.

This includes politically stronger; poverty, disengagement from the labor force, and high inequality are not good for a country's political stability.