I imagine the best case scenario for the US is that it becomes like post WW2 UK.

Worst case, the wall along the southern border everyone is so obsessed with will be used to keep US citizens out of Central and South America in >70 years.

There is usually one showstopping global power. It's been that way for all of history. I haven't seen an argument for why that would change suddenly ... unless we assume the global reserve currency changes to something less centralized.