But dollars in minus dollars out is not the only deficit we're running. There is also a growing deficit of needed infrastructure improvements and repairs, and it will continue to grow whether we add it to our actual balance sheet now or in the future. We will have to pay to repair/replace a crumbling bridge at some point, whether it's today or in 20 years. It will inevitably hit the real debt at some point (unless one wants to argue for private ownership of roads, which is a conversation I'd be happy to have), and our infrastructure will keep crumbling and getting more expensive to fix the longer we wait.
Given that the Treasury can borrow money at historically low rates, and also that there are lots of people looking for work, why would we not choose to borrow now and make the upgrades now? Assuming the benefit to the economy of having better infrastructure outweighs the interest rate (which, to remind you, is historically low and will only get higher), it's a win-win. You get "stimulus" now without a real change in the long-term debt, since money for infrastructure improvements will have to be spent eventually anyway.
The same kind of argument applies to education, research and the like. Yes, we can save money today by cutting investments in the future. But what do we do, then, in the future?