R&R weren't trying to claim that, were they?
Honestly, there are so many variables and dimensions in this system and so little data I'm not sure we should ever be drawing sweeping conclusions like this.
That doesn't mean they were asserting what the grad students claim, that the falloff in growth is unavoidable, which sounds like an extreme strawman.
But their own public pronouncements, and the way they responded to other's interpretations of the results, pretty much indicated that they though they'd found firm evidence that there was some big non-linearity at 90%.
"We show that, contrary to R&R, there is no definitive threshold for the public debt/GDP ratio, beyond which countries will invariably suffer a major decline in GDP growth."
RR's claim is entirely statistical, so of course they're not going to claim that exceeding the ratio always ("invariably") leads to a major decline in growth.
RR's being wrong doesn't mean it's okay to insinuate they hold ridiculous positions that they really don't. That's just compounding public misconceptions.
The former is what RR said, the latter is the strawman being attributed to them.
The response by Herndon doesn't insinuate that countries should disregard debt as a matter of concern. It doesn't even say that debt increase isn't correlated with GDP decrease. The main issue that the paper points at is that US and European governments have, based on this thesis, adopted drastic policies of austerity and budget cuts, when actually over history there isn't such a nonlinearity.
I'm not sure this is right. Rogoff said "back in 2008-9, there was a reasonable chance, maybe 20% that we’d end up in another Great Depression. Spending a trillion dollars is nothing to knock that off the table." I didn't see any policy recommendations in the paper.
The main issue that the paper points at is that US and European governments have, based on this thesis, adopted drastic policies of austerity
_Maybe_ for the US, but the timing doesn't match for Europe. For the UK, the usual candidate for self-imposed austerity, Cameron was arguing for austerity before the paper became well-known.