I am fine with it. It will give me time to let the new features of nis and the new music system really sink in before the amazing graphics are completely usable.
It is a smart move, especially with how unstable/laggy the HTML5 beta currently is. There is no way the current beta can test how the game will run when there are 50k people running only it.
RS3 is the '6th Age' which includes HTML5 and the new game content. "We're releasing RS3, but not ALL of RS3. This way we can keep our promised deadline AND release a non buggy HTML5, hopefully..."