Without the Trust Fund bonds to cash, we would be forced to raise the payroll tax about 1/10 of a point a year -- as average income grows 1-1/2 percent per year -- to maintain the going benefit level (until near mid-century when the retired population is predicted to stop increasing relative to overall population).
Whatever payment method to cover our pay-as-you-go retirement system -- and the present double-trouble arrangement looks clunkier and clunkier as the time to invoke it's clunkiness comes nearer -- we have to raise SOME tax the SAME overall amount to keep paying-as-we-go -- until retiree population finally fills out.
PS. It is entirely possible (even likely?) that, when the FICA gap that is supposed to trigger cashing Trust Fund bonds (w/income tax) finally hits, we may opt instead to run up even more (?) income tax debt (by selling new bonds to private -- i.e., real) owners -- thereby leaving even more debt for our great, great, great, great grandchildren to pay off , in this case to cover the retirement of their long deceased great, great, grandparents. That is the alternate (likely?) significance of the Trust Fund.
Addendum: it has occurred to me that the best way to return to normal S.S. funding (all FICA) would be to pay down the Trust Fund until there was about a 5 year bridge left (a bridge to cover any temporary shortfall in the FICA stream while Congress gets around to hiking FICA -- the only sensible rationale for a trust fund) -- at the point we see the bridge-point coming, we can begin to phase (rather than one-jump) income tax out and FICA in.