The most obvious reply to this is that if there really IS a trust fund, then why can't we put 100% of our contributions into private accounts? After all, since there is a trust fund, we don't need the funds that current payers into the system are providing.
The real problem is that the surplus is being spent on general expenses -- if this can't be used that way, the Feds need a he11 of a lot more $s to pay their way each year.