How to Monetize an API

There's one critical step this leaves out though:

0) Pick a domain with significant costs/regulation/other barriers to entry.

Otherwise your beautifully designed and documented API is a blueprint for developers to go off and build it themselves to avoid you as a middle-man. Unless there is some downside to that, it's just a matter of time until it happens.

The reason the likes of Stripe and Twilio don't lose their business to open source or cut rate clones is that they're doing huge amounts of work that it isn't easy to replicate. (PCI compliance and security for payment processing, hardware and interfacing w/ telcos for SMS)