The biggest challenge however is one that both Apple and Google face: Only a small fraction of the 10 million or so retail outlets in the U.S.â"220,000 at last countâ"have checkout readers that can accept payments from either system.
That is definitely true, but most credit card readers in the US that do not support EMV (aka Chip & Pin or Chip & Signature) have to be replaced if the merchant doesn't want to bear the liability for fraudulent transactions.
The liability for compromised cards is shifting in October of this year (aside from some unattended systems like gas pumps which happen later). If a merchant does not support EMV and an EMV card is compromised or used fraudulently, the merchant is liable.
Many of the new EMV capable terminals are also capable of NFC/contactless transactions. It will get a lot more of the physical readers out there. Whether the payment processors/acquirers support it is a different question.