Well, The way this ruins games is usually the addition of some sort of currency, followed by a hardcore locking down of the game, heavy DRM, heavy anti-tamper, removal/sanitization of mods, and removal of any kind of community control or modding.
This is because heavy DRM and anti-tamper and a real crackdown on community engagement and modding goes hand-in-hand with selling any kind of thing in-game that the community would have made for free... if you can mod things you could just mod coins in or whatever items / stories are the equivalent of the ones you get from coins. Gotta put a stop to that.
Same kind of thing happened when they had the bright idea of selling people cheat codes in the Assassins Creed games. They realized that hang on, people just have cheat codes on PC anyway, without paying, because its PC. So a large portion of the 'anti-cheat' measures in a mostly single-player game were there to stop you from accessing single player cheats/trainers purely so that they could then sell them to you as DLC.