>>82180388The point isn't to minmax returns. If that's what they were good at they would have become accountants instead of authors.
The point is to make enough passive income to retire without ever needing to do any real work again. If you keep shitting out books you risk triggering fatigue, where people lose interest in your franchise, regardless of the quality or popularity, because they've simply had as much as they want of it. If you finish the franchise, you give people closure and they feel comfortable moving on and putting it behind them. You leave them hungry and unsatisfied while vaguely suggesting another meal is coming, all the while letting your publisher and agents capitalize off that continues interest.
And as an aside--authors make sweet fuck all by actually selling books. Publisher deals are typically awful and they pocket the vast majority of the income from the actual book sales. But the intellectual property is yours, and that means you stand to earn a lot (and I mean, a lot) off of merchandising, licensing, tv and movie adaptations etc. The goal of being an author isn't to sell a lot of books, it's to create a franchise that sells a lot of things that aren't books, and in the process cut your publisher out as the middleman. Often it becomes necessary to stop actually writing in order to pressure them into giving a better cut, because it undermines your negotiating position with other companies if you accept a shit rate in one place and not others.