In my experience, people stops playing a lot sooner cause of boredom.
Practical example: let's assume that to maximise a 3 star girl in a low grinding game would take you 2 weeks, including the certain number of battle to get her. That is a reasonable time.
With heavy grinding required, you can't know when you'll get the girl (and random is always bad, as I said before), plus it takes months to maximise her, while other girls keep piling up. This is the hamster wheel effect that kills any will to keep playing a game.