a) Government borrows $4 trillion from the Chinese at 2% annually.
b) Government borrows $4 trillion from Americans, by way of Social Security, at 2% annually.
What's the difference in what the government owes after 10 years? Either way the government has to raise $4 trillion (plus interest) in taxes, borrow it, or print it to pay it off. Either way everyone is in the exact same boat, except in boat b, Americans get the interest. In boat a the Chinese do. (Unless you count borrowing from the Chinese, leaving the Social Security money in a CD, and defaulting to on the borrowed money while paying out SS an option, which it isn't.)
I wasn't suggesting that government borrowing is not a problem. It is. They probably should just not borrow the $4 trillion from anywhere. But that's just a different issue that has nothing to do with Social Security, as many economists have pointed out.