It occurs to me that, if things got sufficiently bad, they wouldn't be able to launch anyone on those one-way trips offplanet. Apollo-era rockets would be enough, especially if there are galactic passenger liners waiting in high orbit (which would make sense) but Apollo-era rockets still require a pretty hefty industrial capacity.

Also, how screwed you are by losing access to the ocean and to intercontinental trade, depends very much on where you are. The USA is in trouble initially because we went and exported all our manufacturing base to China, but could probably recover with native resources. Japan, on the other hand? Completely screwed.