I think that, since PKing is going to be restored, RuneScape will become one of the longest-lasting MMOs in the history of video games. After Ultima Online and EverQuest have faded away into insignificance I expect that RuneScape will still be a thriving game. As far as I am aware of it's the oldest MMO that's still growing.
UO and EQ are still growing. Plus, rs2 != rs1.