Holding physical cash isn't free. You need a secure safe for storage, which costs money. If you still use a bank account for any electronic transactions you will need to physically travel to the bank to deposit or withdraw cash as needed. Not only will this travel itself cost money in most cases, but the cash you're transporting is at risk of theft during these trips. Large cash transactions require extra paperwork and are subject to scrutiny due to money laundering laws.
If you're running a business that has a big account balance, you'll probably need to make your own cash vault and hire guards to monitor it if you choose to leave the banking system.
While all this expense will probably prove worthwhile if interest rates go too negative, it's not worth it if we have something like -0.25% interest.