Why does the world need the US, if all they do is overconsume at the expense of everyone else?
The US gets the world's stuff; what does the world get in return? Paper Dollars, which will never been redeemable.
An analogy would be a group of people shipwrecked on an island. Under our current model the Chinaman does the fishing, the Japanese guy collects the firewood, the Taiwan guy builds the shelter.. and the American gets to eat the dinner. Based on our current system, we would take a look at this and say "look! This economic system depends on the American! without him none of these other poor islanders would have any jobs!"
That's how screwed up the current system is, and it is a relic of a monetary system that was put together when the US was the world's richest country because they produced far more than they consumed and exchanged the rest for gold. Today it is the exact opposite; the US has gone from the world's largest creditor to the world's largest debtor nation.
I agree that demographics are also an increasing problem - why do you think it will only last 30 years? The ongoing costs of healthcare and pensions will only grow.