Obviously if retailers accept BTC the price will drop in the short term as those retailers dump btc immediately for dollars.
It's more complicated than that.
You'd be right if the payment processors got dollars by selling on the exchanges, but that's not their only source of fiat.
Payment processors sometimes have access to dark pool liquidity. There are investors who want to move into Bitcoin either gradually or else in amount larger than what the order books of the exchanges can easily handle. Those investors go to payment processors and agree to supply them with dollars in exchange for the received Bitcoins.
For that reason it's not obvious that retailers accepting BTC will affect the exchanges. It all depends on how much dark pool liquidity the payment processor those retailers use can access.