> What happens if economic ties between coastal regions of major trading partners become greater than cultural ties within nations?

Until you can fix your toilet, repair your house, teach your kids, pave your roads, and have your meals cooked for you remotely, I don't think this is a valid concern. The upper "coastal" class depends on local, blue collar labor that can't be outsourced (with current technology).

Rich folk can move to the suburbs but everyone will need to coexist within contiguous geographic states for the foreseeable future.