they have only now been dethroned as the default credit card processor despite being awful to work with when I tried to integrate their API circa 2010
Setup was harder, and the integration wasn't the same, but it was before PCI, so integrating with CyberCash meant you could own the whole payment flow, and have a better experience than PayPal, where you send the user off, and when they come back, PayPal may not have confirmed payment yet.
> The company filed for Chapter 11 bankruptcy on March 11, 2001. VeriSign acquired the Cybercash assets (except for ICVerify) and name a couple of months later. On November 21, 2005 PayPal (already an eBay company) acquired VeriSign's payment services, including Cybercash.
https://en.wikipedia.org/wiki/CyberCash
so in a way, PayPal ate a lot of the other payment processors
Temporarily. Once people see you're doing well with no competition the competition will spring forth, no matter how good you are. If you're also doing things badly those competitors will eat your lunch.
That isn't a reason not to launch when your product is very basic and scrappy. You don't need to make it good, and definitely not perfect. You just need to go fast.