Having such a permanent law would be constitutionally iffy. The power to borrow money that is part of the US national debt is specifically reserved for Congress

Having such a permanent law would be constitutionally iffy. The power to borrow money that is part of the US national debt is specifically reserved for Congress, and this is essentially part of what makes the United States a democracy in the first place: only the body made up of the country's elected representatives can tax and borrow in the name of the country. (Other countries have similar restrictions, though not necessarily debt caps; for example, in the UK Parliament must passs a budget each year or the government effectively cannot finance itself and continue operating. This would probably not work well in the USA because Congress has trouble passing budgets reliably.)