At the moment the federales have added federal employ retirement funds to the ledger of things they've borrowed.
They've done that before.
The federal government is out of control. The way to get it back into control is to force it to cease spending money it doesn't have on processes it doesn't need to handle.
SS is recently cash negative, but you are right that it has, on a cashflow basis, over the years helped conceal the true deficit. Nevertheless, the government wouldn’t have defaulted on the debt without the SS cashflow.
Leaving cashflow aside, SS contributes a massive off-balance sheet liability. SS and Medicare may have unfunded liabilities of as much as $107 trillion over the next 40 years or so.