Negative interest rates are outrageous. Assets that lose value over time are not worth keeping, they are meant for using then. I think if times come when people will lose money for keeping them in banks instead of gaining (at least a little) it will be a demise of current banking system.
I don't think you understand how inflation works. Keeping your fiat in a bank results in a net loss as long as your interest rate is below the rate of inflation. With interest rates at a severe low for the last several years, the best rates you can get will give you an introductory offer of 1.5 - 2.0% APY, if you shop around. That rate will usually be cut to <1% after a year. With inflation sitting at around 2 - 3%, that means although the amount of fiat in your bank account goes up, the net value of your assets (your purchasing power) goes down.
Any interest rate below the rate of inflation is already a net negative. If you want to earn money you need an interest rate at least above 3% APY, which you will usually only get from longer term investments such as bonds or mutual funds.