With bank runs and related negative trends on the rise. "Cash is king" sounds like a great policy to me. Certainly this is the wave of the future. Only bitcoin beats cash. Gold, silver and precious metals are too clunky, unsupported and cumbersome to efficiently conduct transactions. The same can be said of a barter system. Cash is one of the best options we have atm.
I'm curious if merchants would actually use their right to demand cash only from their customers? I think a business that tries to do that would quickly realize that they are losing customers, because many people already barely carry any banknotes and don't want to withdraw money from ATMs, since it costs them additional fees. Plus they need to count the change from every payment to make sure that the cashier didn't make a mistake.