HTTP is used for far more than just browsers. how broad is the current adoption of HTTP 2.0? Will there ever be a time when all HTTP clients, servers, proxies, etc will have upgraded to 2.0 - clearly that's never going to happen in the foreseeable future so we're stuck in a mishmash of supporting HTTP 1.x and HTTP 2.x clients and server software indefinitely - similar to XMPP extensions the article refers to. It's obviously much faster to innovate and update all clients when you have control over a centralized network and auto-updating clients.