We've had "emergency low" interest rates for 8 and a half years now, so stating that these rates have "done their job" seems like a bit of a stretch.  At best it is sort of like the old saying about the broken clock telling the right time twice a day.

The real problem I see is that these emergency low rates have lasted nearly throughout an entire economic cycle.  It will take a long time before rates return to anything close to "normal" levels and we'll almost certainly hit the next global economic slowdown prior to that point.  Economic cycles are, well, cyclical.  Right now we are pretty close to the peak of the current cycle if historical trends are anything to go by.

How long do we have until the next slowdown?  And how severe will that slowdown be?  My crystal ball isn't clear enough to answer either of those questions, but I do know that the slowdown WILL come and nearly a decade of "emergency low" rates will, in "hindsight" to some, be seen as a problem at that time.