Widen your category definition and you'll see it.
> consider how much it limits the viability of an online business to not accept credit cards
For the vast majority of online businesses, accepting exclusively crypto, or exclusively bank payments, would result in much less business. Orders of magnitude less business. They are not viable alternatives for the vast majority of purposes.
I would add a qualifier to your statement: for the vast majority of global online businesses. An online business serving a single country could make use of country-specific payment systems, which are often very popular. If a Brazil-only online business accepted only payments through PIX and boleto bancário, that could result in less business, but not orders of magnitude less business.
Along the same lines, I qualified it as "online businesses" because there are still some brick-and-mortar businesses that require cash. It's still limiting your business, but less so, especially if you're in a context where people expect many businesses to be that way (e.g. a farmer's market).
But for online businesses that aren't country-specific, which I'd argue is the vast majority of online businesses? You accept credit cards or you get a lot less business. (And the next-most-popular option, PayPal, does even more of this kind of thing than Visa and MasterCard do, and much more capriciously.)
