You make it sound as if that swap was something that happens automatically - but it is not. It is something easy to do and it has been done so far consistently - but there must be circumstances where they stop doing it at least temporarily (a stalemate in the Congress or something).
It may do in the US Congress due to interaction with the debt ceiling legislation and the like and, of course, it being the US it will no doubt end up in court.
Here in the UK, by contrast, it is a 'standing service' on the Consolidated Fund that is governed by two acts of Parliament (The National Loans Act 1968 and the Exchequer and Audit Departments Act 1866). It is entirely automatic and mandated by statute. Nothing can stop it happening. Nothing can even delay it happening - short of a technical failure in the computer systems. Gilts will always be deleted, and the replacement bank reserves created by the Bank of England on the date of maturity.
