I can tell you right now that I would hesitate to use a 3rd party to handle all transactions. Why pay the fees and give up anonymity for a little convenience. Sort of defeats the purpose of cypto's to begin with. Although I can see 3rd party being used for petty transactions like coffee or fast food where even 1 minute is too long to wait.
Retailers usually don't have a choice.
They support whatever payment methods they have to attract customers. If the choice is between paying a 2% transaction fee or loosing 10% of your customers then they pay the 2% fee. On the other hand, they could start to offer discounts for crypto currency payments handled by one of the crypto-payment processors like Bitpay, Coinbase or Scharmbeck.
In that case, we could see a gradual migration to crypto-currency transactions and away from the traditional payment processors, at which point Visa and Mastercard etc will simply start making corporate acquisitions of crypto-currency payment processors and support those type of payments in addition to their existing services.