from my experience from past games and business.... ive been running different business online since 1998 im pretty sure this game is no way near it's end.
Looking on the numbers im pretty sure this game and the market will continue for lots of years to come.
I'v helped smaller game companies with their network structure and they would kill for such a strong brand as runescape.
Old games like RS need to re-invent themselves once and a while to keep the brand somewhat fresh... sure they might lose some old dogs... but they will regain more new players in my opinion.