They're not arbitrary, but they're not razor-thin, either. If we take a quick look at profit margins (net income over revenue) for 2017 as a proxy for fees relative to service costs, Visa was something like 36%, which is pretty good (for context, Google's is ~22%, Apple's is also ~22%, Citigroup is ~25%, Mastercard ~31%, Paypal ~13%, Facebook is just under 40%).

It would definitely be a big shift from the "support" they offer now, admittedly, but if they can break the Visa/MC duopoly at scale, I think you could make a business case for it.