The number of those areas will grow though, so there will be less places to move to.
In the current landscape I think very few people on a global scale has any sympathy with Americans being affected by the changing climate.
"Hypothetically, if we were to jam every human on earth into a space the size of Texas; it would just have the same pop. density as Manhattan" is not remotely the same as "in practice, all of humanity could comfortably live in a space the size of Texas with the density of Manhattan."