> The idea is that you never have to send the template again after that.

This implies or assumes that sending the template again is a problem. In practice, it's not. HTML compresses well and the difference between the compressed JSON and HTML response is often on the order of hundreds of bytes. That assumes equivalent data content which, as I've said above, is often not the case. JSON payloads tend to be larger and we need more of them to render the same UI.