We have representatives but gen z does not. How is it fair to spend their money before they are even old enough to have a say? Was the idea of pushing debt onto future generations so unthinkable that it is a constitutional blind spot?
"No taxation without representation!" was a slogan of the US war of independence, but it was never put into the Constitution. Indeed, the residents of the District of Columbia and Puerto Rico now pay Federal income tax, but are not represented in the Congress that sets the taxes.
National debt has some of the same economic effects of a tax, but it is not a tax in form nor in constitutional law. Constitutional limits on tax legislation (such as the rule against unapportioned direct taxes) do not apply to congressional borrowing.
"Representation", in the sense you mean, has never been applied to representatives of future generations, or even of people not yet of voting age.
Was the idea of pushing debt onto future generations so unthinkable that it is a constitutional blind spot?
Quite the reverse. Borrowing was initiated quite early, while many of the framers were still in government. In several places in the Federalist Papers, Hamilton emphasized the need for an unlimited power to tax, for, among other purposes, paying any national debts, and made it clear that such debts would be uncured, from time to time. See particularly Nos 30 and 43.
And when he became the first Secretary of the Treasury, Hamilton embarked on he famous program of encouraging manufactures, funding needed "improvements" (of roads and harbors, in particular) largely by borrowing.
In short the Constitution does not contain any requirement such as the question suggests, and was never intended to do so.