Going back to the server for 100kb of HTML and reloading the entire page, as opposed to fetching 10kb of JSON and instantly updating it in place, is a very 2002

Going back to the server for 100kb of HTML and reloading the entire page, as opposed to fetching 10kb of JSON and instantly updating it in place, is a very 2002 way of doing things! The difference between streaming 100kb and 10kb is small when compared to the connection latency itself. Even on a low-end LTE connection (5mbps), 100kb is 20ms, whereas the latency of a cross-country (US) transit is realistically ~50-100ms. The difference decreases further when you enable server-side compression and start doing fragment rendering, which tends to remove the penalty for HTML payloads. The argument for react-like technologies is not connection latency, but rather, the hope that you can avoid synchronous connections in the first place by moving your view logic client-side. And truthfully, few client-side apps are written in a way that would minimize payloads anyway. Most people are doing it because they hate working with the DOM and AJAX and server-side frameworks in a mishmash of web technologies, and perceive these client-side frameworks as easier.