Spending has been "unsustainable" for 40+ years, ever since Reagan gave up on trying to get his own party to reduce it. I'd say that's a pretty good indication that the status quo was working well enough.
Musk and Trump are not concerned about debt or spending. If they were, Trump and Republicans wouldn't be rushing a billionaire tax cut bill that will add tens of trillions of dollars to the national debt.
If Trump cared about debt he wouldn't have run up $8 trillion in debt his last term with PPP helicopter money and more billionaire tax cuts.
Or proposing to create a PIF-like Sovereign Wealth Fund while gutting USAID, an organization with approved budget for many many years that actually provides assistance efforts globally.
Or proposing to invest $500B on AI development, which, coincidentally, just so happens to be the amount Sam Altman asked for to develop Stargate.