If you mean holding in a bank. Some European banks are already charging negative rates for large deposits. They have to, because ECB is charging them. You see, all money is either physical or credit or excess reserves deposited at ECB. There is no electronic money outside of ECB system.

If you mean physical, it's also costly. Vaults, protection etc. And to make this option harder, they cancelled the 500 Euro. If people starts piling up 200s, they will cancel that too.