I've been maintaining a free app for over 6 years now. It's a lot of work. It's not like you have crunch time, but it's like you can go from just doing light maintenance to suddenly needing to drop 20 hours on it within a week on a dimes notice if there are major changes to certain dependencies. On top of that, the consistency is difficult. I will get bored and not work on it for months but I still need to keep up with the latest changes or else compatibility might break.

Having monetary incentive really helps with the boring work. The work that you need to do to keep the app going. And not necessarily the feature work which is usually a lot more fun.