Now - take this same line of thought through to the stock market. How much stock does this generation have tied up in their 401Ks and other accounts - what will happen to the price of those stocks as they go to liquidate it to enjoy their retirement years?
The difference is that foreign investors can buy US stocks thereby keeping the values up (not guaranteed, but it's possible), but the US FedGov can't levy SS payroll taxes on foreign workers.
So the pool of potential buyers of stock is equal to the population of the first world while the pool of SS payroll tax payers is limited to those earning wages in the US.