I agree with your assessment for the most part, but I'm not sure your rent critique is complete. Sure, transaction processors would be essentially taking a non-governmental tax in exchange for the service of processing our transactions, but a.) that already happens and b.) there's nothing inherently wrong for charging a fee for a service which the government no longer provides. The issue, I think, comes along when you consider that this incentivizes these processors to process more transactions, and at a higher rate, which you come closer to in the cost argument. (Incidentally, this is also why I'm fundamentally opposed to privately owned prisons - their owners have an interest in increasing rates of incarceration, and tend to act based on this interest.)

One other thing: 'cashless society' isn't exactly the same thing as 'cash prohibition'. The mere fact that the government no longer produces physical currency has no bearing on the ability of private citizens to produce, assign value, and use their own cash in transactions.