Cash is the original anonymous "crypto"-currency. Still the preferred tool of money launderers, drug dealers, pimps, black marketeers, and other neerdowells everywhere. Also, offers real anonymity with no blockchain records.
If the same nodes repeatedly 'touch' the same serial numbers you can infer a relation.
The only downside to cash is a central authority controls it and can devalue if they so choose. Other than that, I agree, cash is great.