You assume too much
Social Security may be privatized and on its own by then...
If not we'll have deficit spending and/or means testing.
Neither would cause an economic catastrophy.
My bet: The US economy will be at least twice as large as it in now in 20 years...And the worrld ecomony will grow even faster (Assuming there aren't any large scale wars that get in the way).
with all that said, we can't predict the economy 12 months in advance, so looking out 20 years is a real longshot.
Scott Evil