I've yet to hear true success stories in this space; to me it just seems like wishful thinking (on part of the marketplace and on part of API developers) about a second app boom; notably, in the hype cycle, the API boom has largely been superseded by the Chatbot boom, which is functionally very similar, although the gatekeepers are different.
That is not to say you can't make money this way. Perhaps it's useful for hobbyist-level projects, or cut-from-the-same-cloth wrapper APIs that hardly offer anything unique but at least one sucker signs up anyway -- the same as with apps, really. Is this actually a thriving market?