But having rides closed doesn't cost them more money than having them open. while the engineers potter around from ride to ride, people who would be going on the closed rides either go on other rides, play the carnival games, or buy food, drinks and souvenirs. If you think what the guests end up doing instead, then you'd need to question why you'd pay more to see less money through in-park purchases.
also, there is no evidence people don't return, is there? I mean, attendance has been increasing for years, and doesn't show signs of stopping anytime soon, and unless there is some mass exodus of fast track users in one go, then I don't think fasttrack makes a significant difference in wait time, especially on the high throughput rides like the swarm, where the thing that slows down the line is usually the baggage and batching being messy.