I have a lot of (my own, highly speculative and uncertified) thoughts on this, and if you'll indulge me in some cynacism I'll try to highlight what doesn't feel quite right.
If you're curious about how this is developing, try Black Desert Online. It's a really remarkably polished game that is truly unique. It's a PvP action fighting game encapsulated by an immersive RPG with a load of different ways to play the game.
It's almost terrifyingly well crafted.
There is a startlingly well designed progression system pivoted around raising your attack and defense rating to fight against other players in the various fields of battle. In every one of the sub routines within this intricate Skinner Box there is an efficiency modifier which can be metered through the use of USD,
All of these sub routines inevitably connect at the central purpose of pumping black stones, or as I'll refer to them, probability stones into your gear with a chance of either raising their rating, lowering their rating, or in certain cases destroying them completely. Sometimes the undesired outcomes can be negated through further remittance of USD, for this and certain other mini-games as well.
There is a tightly controlled exchange of goods through a central system (which collects a tax unless you pay for a buff every month), with only a small number of consumable items being transferable between players.
You are utterly economically isolated in the game.
This is not a new phenomenon, but I think it's clearly evolving.
Casinos were built on it, and so are many of these so-called "Korean MMOs".
Pearl Abyss has enjoyed massive success and, to my dismay,
has purchased CCP Games, developer of EVE Online.
https://www.gamasutra.com/view/feature/131494/behavioral_gam...
I hope these system designs are less insidious than my experience has led me to believe.
Edit: I'll add that I've read supposedly regulatory action is being taken, I read somewhere that China was or is imposing a rule for disclosure of probabilities in at least some circumstances. Not confident on veracity.
Curiously, in certain regions there are apparently varying or no limits on purchase of "cash shop" items, while in others (like NA) there are. Also worth noting that within the game, players who've paid the most into the cash shop are called "whales".
Edit2: Last I looked they boast a player base in the 2-3 million range.