> My friend then retorted that the 15 Trillion was
> "merely a difference in accounting." The 15 Trillion
> being paid in advance was merely the present (or near
> present) cost of later benefits.
"Merely"? Suggest this to your friend: You want to end SocSec right now. You will issue everyone a check for their input into the system, plus interest as if they'd invested in inflation-indexed treasuries, and let them invest it however they would like, washing the government's hands of it.
Regardless of the wisdom of this plan, ask your friend where you will get the money to issue the check. If it's "merely" a difference accounting, like having the money in savings and transferring it into checking, then it should be easy to find $15 trillion. Oh, and no levying a one-time $50,000 tax on everyone in the US. Of course, it's not "merely" a difference in accounting -- the $15 trillion you're talking about doesn't exist anywhere except as a claim on revenues of the US Government -- revenues which are entirely in the future.
All of your other comments are on the mark, but I don't think it's relevant to convincing your friend that the present value of future benefits of SocSec is not "mere".
-- "The more I practice, the luckier I get." -- Ben Hogan