If the physical rewards were enough of a draw in and of themselves, I think we could reasonably expect there to be a market for games that don't involve money. For instance, I could imagine smaller, cheaper slot machines sold directly to players, to take them home and feed them coins all day, just to see the pretty lights (I mean they could get their coins back at the end of the day).
The fact that there doesn't seem to be any serious interest in that sort of game machine tells me that the monetary rewards are the primary purpose of play.
I guess I have and I totally believe games like Candy Crush are exploitative, perhaps even at the same level as gambling.
I remember reading an interview with Tarn Adams (I can't find it anymore) where he pointed out that games that make people perform repetitive actions for some in-game reward are exploitative. At least that was the gist of it. I think he had Cookie Clicker in mind- although maybe I misremember that, because Cookie Clicker is more like the opposite, a game that doesn't hog up your time (it plays itself).
In any case, I think what he said is true. A game that makes you want to press the same key 100 times is just stealing your time. There's always rewards, of course, but if you think of what you could be doing with your life in the time it took to press that key 100 times, it starts to sound a bit pathetic.
The worse thing about this for me was realising that many of my favourite games are exactly that kind. I play lots of shmups, for example. In fact I used to play shmups since I was a kid, when you could find them in arcades and you had to pay to play. I played some of them long enough that I got to the point where I could finish them with one or two coins, so I could say I had some sort of control on the game, but the truth is that each one of those games I liked as a kid was specifically made to draw me in and hook me up, by plugging into my reward centres. And there's no escaping, either, that no matter how good I got I couldn't play if I didn't pay at least one coin.
So, yay, I do 100% think that there are other games besides slots that we're used to think of as bening, but that are also designed to hook you in to some useless rewards and to make you waste your time and your money.
And I haven't even touched on my one true addiction: Magic the Gathering. You don't want to get me started :)
Already exists, in a way, and are quite popular. Check out the slots games on the Apple and Android app stores; the ones that are created by subsidiaries of slot machine manufacturers (and therefore can make their slots in the likeness of real machines) rake in an astonishing amount of money, despite the fact that the player is just playing for virtual tokens rather than real cash payouts. A couple are in the top 10 by revenue in both app stores.