I agree there is a burden (inefficiency/deadweight loss) because of government spending made possible by borrowing, ignored by Krugman. But that is in addition to what people mainly mean by the burden, which is the actual amount of the debt. In the case of default, then the bondholder would bear the burden.
Actually, I was thinking perhaps it is correct to say the burden is limited to that inefficiency and is borne by present generations, but only because the government can continually roll over the debt. The government is always taxing, of course, but for the foreseeable future it is also able to borrow to pay current bondholders. So you have more people voluntarily purchasing bonds (creating that additional inefficiency due to government using resources). Until there is some kind of end game with the debt, is that burden postponed? Is that consistent with what you’re saying?