Based on floor space alone slots would be the biggest money spinner for casinos. Just think of everyone crammed neatly next to each other just pumping in coins. What a disaster for the player. I would love to meet someone who is actually beating them with AP or some kind of legal method. That's one thing I would easily pay for if it really worked on a wide range of machines, jackpots or whatever. Cant imagine anyone selling it if it worked. And cant imagine it would work for long before the casinos noticed.
I once saw a transaction log script like a grocery store docket that came from a slot machine. The tech probably forgot to take it with him. It was a log for about a month ad it had something like $500,000 pumped into it. Then the casino probably earned $20,000 it just a month. A small square of floor space and not a very popular machine either. That was years ago so it would probably be double in today's money.