You could probably say the same about Stripe when they just launched. Authorize.net was doing billions in revenue and stripe just re-invented them with a better suite of APIs.
Truepill is an API for pharmacies that is doing seriously well and is under-hyped. The API-ification of things is a serious boon for developers everywhere.
But financial services (payments) and travel are just two verticals. There are vertical software solutions in healthcare, other aspects of FinTech (lending/wealth management), retail that can probably be re-vamped.
The question is what other legacy providers other than Authorize.net/Sabre exist. Where do we find software solutions like this?
Your missing the part where Stripe also bundled the payment processing (Stripe uses Vantiv AFAIK) and the merchant account (which you need to get from a Bank). Stripe's innovation was to simply flatten the stack across gateway, processor and merchant bank account.
Stripe has an incredible API, but it's only one half of the equation here.