Ask HN: Making Simple Apps Profitable?

I'm wonder whats the latest in revenue models for simple software apps and how to sell them? Let's say the app is a EXIF editor to process bulk images. It's a simple tool. How can one make money off of it? A subscription for such a simple tool doesn't make sense for it to be an ongoing cost. Donations are unreliable. Further upgrades/features to the app, is a possible one. One time purchases make the most sense, but aren't conducive to long term growth/improvements. What are your thoughts and experiences?