Now, I said open source, but that doesn't mean free. Even if the source was available, it would make the most sense if they kept it "non-free", as in, no commercial usage of the source may be allowed.
People are free to run their own servers, but the Jagex ones will be "blessed".
The only problem I see is if some group in a country outside of IP legality were to take it and try to monetize it.
Alternatively I hope one day Jagex has a "hail mary" and releases the source when they think the game is considered commercially dead.