The problem with VISA-MasterCard duopoly is that in many markets the intermediaries (banks plus VISA/MC) skim off 2.5% off of the transaction between the mercant and the customer.
Amex takes much more (~4%?), so the problem is even worse there. Of course, they can and do bribe the customer with part of that fee so that it seems attractive, but from the wider perspective it's a huge overhead for no good reason, and if every transaction was like this (i.e. if Visa-MC was replaced by Amex) then that would be a big drain on the economy.
Amex takes much more (~4%?), so the problem is even worse there. Of course, they can and do bribe the customer with part of that fee so that it seems attractive, but from the wider perspective it's a huge overhead for no good reason, and if every transaction was like this (i.e. if Visa-MC was replaced by Amex) then that would be a big drain on the economy.