Adult 'games' are primarily entertainments, they may be called games, but really they're not. For most players they're wish fulfilment scenarios, where they either see themselves in the role of the protagonist or enjoy controlling them and forcing them to do stuff that they wouldn't normally do. As such, unlike games it's not necessary for them to have an ending, even if they inevitably will. Yeah some people get bored eventually and want the game to reach a satisfactory conclusion, whereas others are quite happy for their favourite characters to carry on ad infinitum, even if they're largely going over the same content again and again.
So I don't agree that games are artificially prolonged simply to earn money. They carry on because a majority of the people who support them, want that to happen. And if you don't support them and don't agree with that, well you're not one of the people that Devs are primarily listening to and basing their decisions on. Yes, Devs make money from this, but they also get a lot of pleasure from satisfying their patrons. And maybe that doesn't satisfy everybody, but it does satisfy the people that matter most to them. It's overly simplistic to see everything in terms of money.
