So make it a 2 sided market, unify the processor into one of the sides. In other words, either a merchant co-operative or a consumer co-operative. In this case, a merchant co-operative seems a natural fit. The merchants jointly own the co-op, and get a refund of their fees proportional to the profit of the co-op.

And you get the consumers on board the standard way: by bribing them. So something like a 2% rebate. So the merchant fees stay at a similar rate, but the merchants win in other ways because they own the processor.

Nobody's going to become a billionaire starting a co-op, but an executive in a successful financial co-op would pull in a multi-seven figure salary, which should be sufficient motivation to interest the startup folks.

And Y-Combinator would have to loan money to such a startup, they couldn't buy in. But it's in their interest to do so, given how much of the Y-Combinator portfolio is dependent on credit cards.