I think there'd still be a use for credit cards in that scenario given that the US payments infrastructure still isn't as good as it could be (especially around security). I probably would just stick to one credit card from one issuer though.

Of course, this all assumes that interchange is actually capped on credit cards. I highly doubt that'll happen any time soon (if ever), but I can see merchants gaining the right to reject higher-tier cards while keeping the ability to accept debit and lower-tier credit cards. Who knows how that would affect things.