In Canada the merchant fees have been going down by a few basis points every year with no impact on rewards. 2.37% is around the same that VIsa & Mc charge on their cards. I doubt there will be any changes in rewards. However if there is a major cut that might have an impact.
I think a bigger threat is the legalization in more and more states for merchants to surcharge customers if they are paying with credit card. It's not only a problem to customers that are hoping to use their cards to collect rewards, but in the long run if there is a cut in the amount of credit card volume used, the banks will need to resort to other means to collect lost revenue.