1) Because of the safe asset problem there is a diminishing return — or even negative return — to QE at some point. In fact, rather than being inflationary, it becomes deflationary.
2) Interest on reserve policy is actually designed to counteract this deflationary — and negative rate inducing — effect. In fact, IOER, or the ability to hold reserves at the central bank for no negative interest cost, shows that central banks are effectively supporting short-term rates rather than depressing them. If not for the ability to hold reserves at the central bank, then rates could very well be negative.
3) The crisis is in many ways a deposit crisis not a debt crisis. There are simply too many deposits seeking principal protection and not enough safe assets to protect against capital destruction by negative rates.
4) Negative rates are a function of global abundance (brought on by technological advances), and a trend that cannot be stopped even by the strongest central bank — unless society regresses backwards (like many goldbugs would seemingly desire). For rates to stay positive we have to hoard almost everything in the world form the people that need it, if it is to have value. The artificial scarcity tactics that have been used through the ages to achieve this, are getting harder to execute because of technological liberation — which is enabling the emergence of collaborative economy which bypasses rates of return.
5) Central banks taking charge of digital money and issuing it directly to consumers is one way to ensure deposits can always be protected from negativity.
6) Value in the capital system, and our definition of growth, is very likely being transformed as a result.
7) Greater efficiency and abundance may also eventually lead to the end of arbitrage.
Here is more. You will find that differs from the perspectives usually expressed here (most of all #4), but it is always good to pass along contrasting points of view.