As you don't know whether Discover or Amex is bigger? Why does that matter? And Paypal is not a credit card. You can link it directly to your bank account and pay with it as the transaction processor. That's a big part of why it's so popular. Might as well complain about bitcoins.
The point is that referring to Visa and MC as a duopoly is inaccurate. There are other ways to process payments besides through the credit card system.
