The Federal Reserve shouldn't go into negative interest rate territory
If you're planning to go negative and you manage to slide down to -4% or close to it, you have a huge risk of making the banks go out of business and people generally take note of it much before it happens, which results in bank runs. This is exactly what had happened in Japan. People had started buying safes (lockers) to keep all of their cash. Not letting people do that, would be way too authoritarian and a violation of people's liberty. Not being able to have control over your own hard earned money is equivalent to slavery.