Actually, negative interest rate policies have been implemented quite a few times in Europe. The Swiss did in the early 70's, Sweden and Denmark both did for a

Actually, negative interest rate policies have been implemented quite a few times in Europe. The Swiss did in the early 70's, Sweden and Denmark both did for a while during the recession, and the ECB did it in 2014 for bank deposits. So yes, it actually can happen. But it doesn't require digital cash.