Yeah, a mass exodus of an entire industry is more or less the only thing could work. Other than regulation from the government, but in the past the USA hasn't really been keen on regulating their precious "free" market.

As to how much money goes through Steam, consider this: Where I live, most banks only give out Visa cards for free, with alternatives technically available but costing money. So, if you have a bank account in my home country and use a card to pay, it goes through Visa. I'm fairly certain that we aren't the only country where this is the case. This poses the question: Does enough money flow through Steam for it to even be noticable compared to multiple countries worth of purchases? Maybe? Probably not though. But should enough companies simultaneously decide to stop using them, it'd at least give competitors a chance to take over.