In the long run the player base is going down and more gold is brought into the game than is consumed so logically it will go down until the end. There may be spikes along the way like summer time, mass bans on stakers, amazing new content, etc. There can also be large troughs from duping, school starting, new gold sources, etc.
Cool graph of RuneScape playerbase:
http://www.misplaceditems.com/rs_tools/graph/?display=avg&interval=qtr_yr&total=1