Thanks for the great question. The idea for RESTful JSON grew from couple of directions. First, it captures what companies are already doing with URLs in their APIs. We noticed that Stripe had a
url in their API and looked around to see the others doing this as well. It seemed like a good idea to give a name to what was already there in a lot of popular APIs.
Second, it came from our own experiences of trying to help others move toward building hypermedia APIs. Many times, it was a hard sell to convince people to move to a hypermedia format because it meant learning a new standard, adding more libraries and tools, and, in many cases leaving their existing tools behind. It also required their API consumers to adopt tools and learn a new standard in order to use their API. We felt there was a gap for a small, pragmatic pattern for using hyperlinks in JSON, so the idea was, just add some links to what you already have.
So we saw RESTful JSON not as a replacement to standards like JSONAPI, but as a means to make it easier for people to get into hypermedia. There will be situations where it makes sense to go this route, and there may be others who are looking for a more feature-complete solution like JSONAPI. Either way, we are hopeful people start using hypermedia through RESTful JSON or JSONAPI—whatever works best.