RuneScape private servers used to bring in tons of money. I helped manage one when I was in my early teens, and I can confirm the owner (who was only a few years older than I) was bringing in mid six-figures annually.

Then of course there was the rampant gambling. The founders of online casino Stake and streaming platform Kick both started their “careers” in RuneScape gambling. IIRC they invented “staking” which was a method of gambling gold against other players, before they were banned. But the gambling economy in RuneScape used to IRL mint millionaires for sure.

Staking was a built in feature of RuneScape in the duel arena before it was removed. They did not "invent" it themselves.