The fact that China just announced it was banning all crypto currency trading is all the evidence we need.
Two key issues concern me.
1. If cash is banned, and absolutely every financial transaction has to go through credit/debit cards, or some other electronic transfers of funds, we lose the remaining privacy we still have of transactions done in cash.
2. It will be much easier for people in power to manipulate our money, if absolutely all money/legal tender, is legally only allowed within a government monitored account of some sort.