Except that when you follow all of REST, including HATEOAS, that part of the API documentation is FREE. And the caching is FREE, because its already been designed for you. Which is the whole point.
You don't, of course, but it's usually a pretty reasonable assumption that the effort involved in implementing your own (reliable stream sockets|scheduler and memory manager|high level language compiler) will eclipse severalfold any advantage from doing so. If you don't find that the same obtains from using HTTP and hypermedia then that's your position and you're welcome to it, but don't go assuming your experience is universal.
Wrong. I implement network protocols for a living, and I use often use UDP because TCP is not sufficient for the purpose. So yeah, this is the problem: your conceit that your upfront design is going to be "enough" for everything.
In either case, I'm done here.