Unless the Government plans to default on its debt it might as well just print money rather than borrow

There is no strong reason against monetising the national debt or even defaulting on it.

The National Debt is money owed by the Government to any private agency within or outside the country, domestic or international. This can be money owed to private investors, pension funds, insurance companies, foreign countries and so on. There are generally very few restrictions placed against people owning Government debt and it is usually held in the form of Treasuries (for longer-dated liabilities) or bills and notes for more short-term debt.

People, especially politicians, worry about the perceived reliability of the debt, often taking it by extension to be a statement on the fiscal heath of the country (or economy) as a whole. But in reality, many of the biggest economies have borrowed to such a great extent that to repay the debt in the expected fashion - to raise repayments from taxpayers - would now be impossible. The debt is so great that only by means of either continued borrowing or actually printing money would the outstanding debt ever be paid back. In a fiat economy both of these options (printing and continued borrowing) are continually available to a Government that has borrowed heavily, since there is no physical impediment against increasing the supply of money.

If the debt has become such a perceived problem for the Government that heavy tax increases are being considered, it is possible that the Government could instead default on the debt. For an ordinary citizen, the disincentive against doing this is that we would find it more difficult to borrow again in the future; our credit rating would have been affected. For Governments this is not a problem since they, effectively, have access to the printing press which means they can always get hold of money when they need it. The most significant argument against defaulting on the debt would be the moral hazard of defrauding holders of Treasuries who are relying on the assets to be valuable, perhaps for their retirement, which would result. But weighed against this is the fact that taxes are being collected from poor people all the time, including pensioners, so in many ways a default of the Government debt can be seen in this way as a progressive measure. Also, people who have lost out in their savings, as a result of a default in the Government debt might be compensated with the issuance of some new base currency, thereby partially monetising the debt. There is no persuasive reason to keep the debt lingering around. The economic damage caused by Government borrowing (inflation, an increase in prices) is no worse than that resulting when the Government prints money outright and so there is no real reason to be fearful of replacing the Government debt in the economy with another form of currency such as physical notes and coins.

It is only if there is a genuine risk of default that there is a difference between Government debt and actual base currency, and since we can assume that the Government never intends to default on the debt it might as well just issue new currency rather than debt.