Generally you are correct: service is paid for -> stop paying -> stop service. However, Github has a free tier for public repositories. Why can one not convert the repository to public then?

I like how Dropbox handles this. If your paid membership is cancelled and you are using more space than the free tier, they don't delete your files, just make them read-only. You can view or download them, but if you want to upload or update something, you have to buy premium or bring your space use within free tier limitations.

People may put all manner of things in private repos that they don't want to be made public, so github shouldn't just expose them to the outside world.

That doesn't mean they couldn't just offer a button to make them public after the fact, I don't think anyone was suggesting to make them public automatically...

GitHub shouldn't make them public, for sure. But people should.

Because github can’t make that decision. It’s your responsibility to convert it to public. There could be sensitive information in the private repos.

...hence why they should give you that option if your paid account expires. It feels like people are being intentionally obtuse in this thread

Nope, that is exactly what it is. There is no reason not to give read-only or export-only access to private repos if your account is locked. Otherwise just delete the whole thing.