They are already "due" -- Social Security started redeeming them a few years ago.
But, you almost answered your question:
Well, the answer is simple. Taxes.
If we were running a budget surplus, it would be "taxes". But, since we are still running a budget deficit -- in excess of the amount being redeemed by Social Security, it's effectively from new US Treasury Bonds being sold at auction, and purchased by individual and corporate investors.
I said when they are ALL due. That’s is when the full weight of it hits.
Sorry, no dice on saying borrowing is not a tax. Borrowing is repaid via taxes or a slow write off via inflation or a debasing of the currency. Doesn’t matter it’s all taxes which is the government taking wealth.
But you have described a Ponzi scheme.