The actual calculus is that you spent 30 minutes on something that should have taken 30 seconds, but then you're done with it. The "proper" solution is to spend months or years fixing the workflow for complete strangers for free, even though you personally will never get that time back. Yeah, it moves the world forward, but it's not always the best choice on a personal level.

Also keep in mind that most of such charitable work goes nowhere. There is a fair number of projects shaped like ffmpeg or QEMU that have never achieved the critical mass. I've written a number of small utilities that simply went unnoticed because they were never featured on HN or anywhere else. Writing FOSS is pretty similar to starting your own band. It helps if you're a good singer, but it's not enough.