Why don't you want to spend the next year building out an app that you might abandon in the future? What's the downside of working on a passion project and then setting it aside when the passion runs out?
I think that part of it is the opportunity cost of working on a passion project. I have more than one of these types of projects, and I have to weigh them against one another. For example, I could work on these projects and building them out, or at more features to my personal website, or build something new.
So at any given moment, work on the project that strikes your fancy? There's no upper bound on passion projects dormant/active/etc, where the Project Police will come hassle you for having too many.