The problem is it's not a gamble, it's a percentage. Even if 75% of your customers have a Visa, then 25% of them won't and you risk losing that amount of business by not taking what they do have.
Moreover, that still wouldn't give you competition, because then what incentive does Visa have to give you a good rate if they're the only network you could plausibly attempt exclusivity with? It's like trying to solve a lack of competition by creating a monopoly.
Moreover, that still wouldn't give you competition, because then what incentive does Visa have to give you a good rate if they're the only network you could plausibly attempt exclusivity with? It's like trying to solve a lack of competition by creating a monopoly.