Honestly, there is very little difference between MMOs and Pachinko machines, which are widely considered one of the worst methods of gambling in terms of both returns and time. They are, by design, meant to be a constant feedback loop, typically tailored to a specific theme or even franchise to draw you in and immerse you. Once you are drawn in, you make a typically small initial investment which places you into the loop. You play the game and develop a rhythm dictated by the gameplay, music, and even interactions from the game itself, but you are essentially just pressing a button to launch a ball over and over again.
The game feeds you little wins every now and then, but actually winning requires a massive time investment as the stats required to land multiple balls into a true win are stacked against you. Just like with MMOs, the upfront cost is cheap, but once you are hooked into the rhymatic feedback loop, you are either forced to opt and quit entirely or make another small investment to continue the game at the same rhythm. The loop never changes beyond this, and the same can be observed with MMOs.
This is the only genre where periodic payments are considered a definitive part of the gameplay loop and community. Yeah, you can play for free/a small amount, but people with the self-control to do that are not who the games are tailored toward. MMOs are tailored towards the whales, the guys who drop large sums of money to keep playing and winning at the game. This is a genre that is inherently predatory beyond even other vidya, and very little has changed since the days of WoW. As for people who mention that social aspect, well this is what the "multiplayer" side of things looks like to non-addicts:
Btw, you can have social interactions and in-depth gameplay in virtually any genre. Playing games with friends is typically fun regardless of quality. You don't have to go to a digital Pachinko parlor to have good, social gameplay.
