(Skip to the bottom for the TL;DR answer.)
The validity of the public debt of the United States, authorized by law, ... shall not be questioned.
shall not be questioned (or the term
questioned for that matter) occurs only twice in the US Constitution and its Amendments:
Section Six of Article 1:
The Senators and Representatives shall ... in all Cases, except
Treason, Felony and Breach of the Peace, be privileged from Arrest
during their Attendance at the Session of their respective Houses, and
in going to and returning from the same; and for any Speech or Debate
in either House, they shall not be questioned in any other Place.
Section Four of Article 14:
The validity of the public debt of the United States, authorized by
law, including debts incurred for payment of pensions and bounties for
services in suppressing insurrection or rebellion, shall not be
questioned. But neither the United States nor any State shall
assume or pay any debt or obligation incurred in aid of insurrection
or rebellion against the United States, or any claim for the loss or
emancipation of any slave; but all such debts, obligations and claims
shall be held illegal and void.
The first instance in Article 1 of 'shall not be questioned' obviously meant 'interrogation' - as in you literally can not ask questions.
The second instance in Article 14 of 'shall not be questioned' is about as grammatically sound as that comma in Article 2; and economically unsound because if it really means 'our public debt is valid' then it misunderstands what debt is.
Although the Perry v. United States case was mostly focused on the legal backing of the gradual move away from the gold standard, Chief Justice Charles Evans Hughes' tongue-lashing :-) of the government could as easy be interpreted as a lashing of debt abuse. Especially this statement (emphasis mine):
By virtue of the power to borrow money "on the credit of the United
States," Congress is authorized to pledge that credit as assurance of
payment as stipulated -- as the highest assurance the Government can
give -- its plighted faith. To say that Congress may withdraw or
ignore that pledge is to assume that the Constitution contemplates a
vain promise, a pledge having no other sanction than the pleasure and
convenience of the pledgor.
In lieu of a concrete law or a tidied up Amendment, the government can slip around the sides of Section 4, Article 14 regardless of whether constitutional scholars consider the section specific to the Civil War and future insurrections (that bothersome
but neither conjunction) or general to all public debt. Because what someone rates a bond (its 'validity') is strongly decoupled from amount of lending and the amount of repayment.
Why? Well, to summarise and keep this a politics instead of an economics answer:
Government debt is not your grandma's debt. It has a permanent tax base, so in theory it can time-shift its debt obligations indefinitely - without even reducing the bond-rating by simply shifting the ratio of revenue spent on debt servicing versus every other obligation.
The bond owners (at least the short-sighted ones) don't care about that ratio, only that their interest and/or maturity has been paid; and thus the rating / validity of the debt is sound.
But as that ratio begins to void other obligations of the government, some of which are likely grounded in constitutional terms as strong or stronger than Section 4, Article 14, it may result in a reinterpretation of Section 4, Article 14 moving away from the Perry vs United States ruling.
So my answer is: There isn't a consensus amongst constitutional scholars as otherwise this question wouldn't be arising both on the Internet and in Washington*. Such consensus will require either a judgement on Section 4, Article 14's authority and priority versus other Articles; or a revision or repeal of that section of that Amendment.
Monographs by lawyers won't cut it because of the inconsistency that Chief Justice Charles Evans Hughes observed.
* A post hoc ergo propter hoc cheat or an application of the anthropic principle. Other flavours of answer icecream, such as citing competing examples of partisan media bickering are also possible (and delicious).