They need to ban cash first and THEN it will be appropriate.
Even if there was a cash ban, they still wouldn't want to go negative. They would prefer to just pump money into the system.
Also negative rate are a relative term. If the bank charged you -1% interest while there is 2% deflation in the economy, then you're really getting +1% interest.
Likewise if you are getting +2% interest in the bank while inflation was 3%, then you're really getting -1% interest.
