which can also happen suddenly, there is no theoretical limit on how quickly the $2.8 million UpBit HBD could be put into savings

Sure, but even if that happened the inflation from HBD savings would still only be around 0.2%, or $700K out of $10-13 million overall. This just isn't that significant.

You would have to see a dramatic change in supply of HBD relative to HIVE and the proportion of HBD in savings for the interest to really become significant. At that point (or along the way) witnesses should (and I think would) reassess.

Also, I think witnesses might reassess if exchanges started using savings without paying it out to customers. One of the reasons to only pay interest on savings was to support higher APR while reducing the windfall to exchanges