I figure we are good for the next 50yrs even if trends continue. Yesterday's 10yr auction of $40B, went 68% to foreign buyers. That is well above average and shows that at the right yield, there is plenty of demand.
I suspect that with all the talk of entitlement reform, someone will re-float the idea of investing SS funds in something other than US treasuries. Given SS holds about 20% of US debt, that could cause demand for US debt to drop. I hope someone is in charge that can understand that. This is why you can never say never.