Since I do almost exactly that, I agree with the sentiment. I'll go even further on one point: "Imagine if you could just send it the whole “page” worth of JSO

Since I do almost exactly that, I agree with the sentiment. I'll go even further on one point: "Imagine if you could just send it the whole “page” worth of JSON" => why do you even send it as a separate endpoint? Embed it in your page! It works great with a SPA: the first load use embedded JSON, and next partial data refreshes hit endpoints returning JSON with the same format.