I don't think it's about money, it's about human nature. Sooner or later, any project is going to run into production problems, especially if they go on for a long time. And even more so when the development is tied to one person - people are very unstable creatures. Over time, the initial excitement fades, problems in personal life start to mount up, or the project just ends up boring the developer so much that even money can't motivate him.
Someone finds the strength to change their routine and keep working. Someone pauses, and some people simply abandon the project. There are those who talk openly to their patrons, stop payments. And then there are those who simply disappear. Maybe they're ashamed to admit the problems, or maybe they just don't care. It depends on the nature of a particular developer, not on his earnings, talent or popularity of the project.