If you look closely American Express isn’t charging much more than Visa / Mastercard

As mentioned above, there is a rationale for charging a higher fee to the merchants up front. If you look at the breakdown of what merchants get charged by the banks and Visa / Mastercard, it actually adds up and the fees are very similar because Visa / Mastercard have been using banks as clearing houses, whereas American Express has had to use itself for that purpose.

I am surprised there is no mention of the lawsuit brought by the Department of Justice against Visa for antitrust violations for having colluded with the banks for years to keep American Express out of the bank-issued credit card business.

The piece only casually mentions that banks have started issuing AmEx cards. But there is much more "dirt" there on what Visa / Mastercard have been doing for years to keep American Express out of the market.