If the government wants to "unperson" someone, cash or no cash or electronic money, they will just come and collect you.

You having pieces of plastic/paper and/or metal trinkets in your pocket plays will not save you from this.

I understand the apparent privacy angle, but if you analyze it further, cash won't help in the presence of a malicious system. If the government cares enough about you to stop you from using e-cash/credit cards to buy or sell, they will also care enough to not let you roam free.