When you're talking about benefits in a national social insurance system over the infinite future, the concept of unfunded liability is stretched beyond reasonable shape.
It is reasonable to talk about expected benefit obligations in 2030 or 2060, as long as you don't take yourself too seriously. Aggregating all future benefits is nutty.
Unlike a business, Soc Sec's "liability" cannot be called like a loan. Nor is it like a balloon mortgage, all coming due at once. The likely payout schedule is known. The debate is over how excited to get about projected shortfalls 50 years away.
Unfunded liability is conservative crackpot rhetoric, IMO. You could as easily talk about the Gov's Social Security wealth -- the present value of all future labor earnings, from now till Kingdom Come.
mbs