The United States will always be in some amount debt, and that's perfectly fine. Going into debt, in order to invest into something with a good RoI is a non-brainer.
It stops being fine when the debt exceeds its ability to pay - which, given that it is in control of its own currency, never has to happen.
What you should be concerned is investments into things with poor RoI. Corporate crony handouts, tax cuts to the rich, building bombs and dropping them out of airplanes, and then rebuilding whatever they blew up, digging ditches and filling them... All of these have incredibly poor RoI.
It stops being fine when the debt exceeds its ability to pay - which, given that it is in control of its own currency, never has to happen.
What you should be concerned is investments into things with poor RoI. Corporate crony handouts, tax cuts to the rich, building bombs and dropping them out of airplanes, and then rebuilding whatever they blew up, digging ditches and filling them... All of these have incredibly poor RoI.