That lawsuit isn't about lowering the transaction rate, it's about collusion between credit companies to keep their rates in line with each other.

First hand, I've seen a small ecommerce shop have rates lowered a 1/4 percent doing less than $1m/y.

Second hand, Costco, who in the US and Canada just switch from AmEx to Visa as their accepted card, went through long negotiations that resulted in booting Amex for not matching the discounted rates Visa was willing to offer.