Quote from laminustacitus:
Putting the interest rate below zero would be an even graver folly than the ultra-low interests rate now. The interest rate needs to be high enough to entice individuals to save so that there's funds in the loanable funds market for investments. If there's still investment without the savings to support it, then its unsustainable in the long term and must eventually be liquidated, leading to yet another recession.
But all the banks are bankrupt.
How can banks reward savers when they are bankrupt?