Note that in order to prove that the GI Bill paid for itself, you actually have to show not just that people who went to college made more money, but that their education actually helped them be more productive. It’s possible that they made more money because having a college education served as a signal that let them into the ranks of management where more money is made, and that the effect of providing college education to certain people was merely redistributive (if they hadn’t done it, some other group of people would have made more money). I’m not arguing that this happened, I’m just saying that pointing out that people who go to college make more money is not sufficient evidence for the claim that paying for college educations is a net benefit to society.
Bonds reach maturity every day. There is no balloon payment coming up. Just an incremental increase in total payments made. And there is more than one way to bring down the debt. The debt always needs to exist (the banking industry is required to keep a certain amount in government bonds for security). The important thing is to lower the ratio of debt to GDP. We can do that by either paying down the debt or increasing GDP. Either way works. As long as the population is growing (which grows the economy) and we have inflation just holding the debt steady or slowing it down will eventually bring it in line.
As to the OP, some government programs do directly pay for themselves in fees, leases, or interest, but these are rare. Last I read the bank bailout of $700 billion will actually end up costing $34 billion because most of the loans will be paid back with interest. If better managed it could potentially have paid for itself.