I agree. Youth unemployment is a big problem. If you create debt and use that debt to drive down youth unemployment then the concept of "pay for it over the next X generations" doesn't even make any sense because when you are creating the debt you are also automatically writing a paycheck that clears that debt. It's effectively neutral on the economy.

If you have an economy with zero slack (meaning 0% unemployment) then adding more debt does increase the amount of work that has to be done in the future. In practice this means that the least profitable work will not get done and a backlog of work starts piling up. This is not a problem in practice because of technological progress and the subsequent rise in productivity.

Debt does become harmful if it is spent wastefully.

Financing consumer spending is a pretty good example. Since you do not expect to net a greater return in the future the debt is just an additional fee that provides no meaningful value compared to having the money in your bank account before spending it.

Getting a mortgage to buy an existing house can also be wasteful if there is a housing shortage (=more residents than housing). You're supposed to spend that money on building a new house or condo. The large difference between market rate housing and construction cost is not a flaw. It's a financial incentive for construction companies to build more housing. You just have to let them build.

Using debt to finance a war is also extremely wasteful. If you lose the war your economy will no longer be able to support the debt.