Since online businesses don't declare their revenue publicly, it's difficult to tell, but from a general point of view and an understanding of how different platforms and services work and how they earn money, I believe exchanges and casinos will be the top two businesses with most revenue among online businesses. We might think that casinos earn a lot of money, but we can't ignore the fact that centralized exchanges earn a hefty amount as well since they provide a lot of services and they earn money on each service they provide.
I think if a casino and a centralized exchange have the same level of user base, they might earn about the same because if a casino earns money from house edge, an exchange earns from trading fees, if a casino gets money when a player loses, an exchange earns as well when a trader gets liquidated, and if a casino has to pay the winners, an exchange has to pay those who earn profit on futures trading.
You'll be suprised to know how much those one casino business generate annually if they happen to declear their revenue i don't know if I'll say owners of gambling companies makes more money than the Crypto exchanges but im pretty sure those two are among the top 5 industrial in mordern time and owners of those business have made generational wealth for themselves the anount they made daily is mind blowing.
Casinos earn a lot daily about 80% of people lose daily and their are people who spend most of their day gambling and chasing loses, well i might disagree that both the Casinos and exchanges have almost the same user base both come to think of it, i feel the risk in Casino could be more compared to that of those exchanges. Well many country have placed a ban on most exchanges but gambling is world wide.