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?