Ok, so I'm a bit surprised he didn't make mention of network externalities as a possible rationale for currency issuance being a natural monopoly.

I also think that there must be some reason why the monopoly on violence and the monopoly on currency are inextricably linked. There's most likely a game theoretic reason here, where it's hard to sustain an equilibrium where the two monopolies are separately controlled.

Lastly, I don't know much about the history of free banking, but it should be mentioned that even during periods of free banking, they were only so because free banking was sanctioned by the state. So, this probably goes back to some inextricable linkage between the monopoly of force and the monopoly of currency.

But, shrug, I'm far from a monetary economist/historian, so these are just my pleb thoughts.