It's an MMO which puts it in competition with other MMOs the same way CoD is in competition with halo even though the games are really different.
COD and Halo attract the same people.
I'm psyched for ESO, because it's a new game being released with some fantastic ideas. Runescape seems like stuff to me as it has for several years. Even when it was out, World of Warcraft was the obvious choice to play. Runescape happened to be free, so that's the only reason I ever logged into it.
There is no competition. The people that play Runescape are just those who remember the game for what it was, and just like to log in for a while.