That very situation is what Krugman did a lot of his academic work on. So far it's played out mostly like what he and other mainstream economists predicted. You can always say "until it doesn't", but that's the nature of life. We have to go with what we know.
It's not as if Krugman wants QE + ZIRP; he'd rather get out of it ASAP with his actual policy prescription (fiscal policy), but he's had to settle for QE/ZIRP.
> Yet there are many economists, myself included, who regard this view [that QE will be effective and give us a strong recovery] as highly unrealistic, yet support more aggressive Fed action all the same. Why? First, because it might help and is unlikely to do harm. Second, because the alternative — fiscal policy — may be of proven effectiveness, but is also completely blocked by politics. So the Fed’s efforts are all we have.
http://krugman.blogs.nytimes.com/2013/07/27/miltons-paradise...