The only people they'll be killing is for people in rag/hides. RuneScape isn't set up with PvP in mind.
Economy has nothing to do with it. They played previous iterations of the server when the economy was significantly worse, they have played spawn servers. Teams/clans don't pk here because it takes time to set up an account to a respectable and usable point and because there aren't enough people. If there were 3x more people concurrently then I'm sure you'd see activity. Game's economy is fine for being something like 2+ years old at this point?