I have mixed feelings here. I legitimately paid ($1, once, a decade ago) for a calculator app and feel it was a great value (I still prefer it to this day). But then again the free version was fine too and the one-time payment was essentially a donation to the developer for a great app that unlocked... Themes...
I like that model. You get to try the software for free and have a way to thank the developer and get something extra, rather than just a pure donation.
F-Droid has an Android port of the Unix maxima, which is a whole free and open-source computer algebra system, that handles all my heavier lifting. And I'm pretty sure that every phone that I've ever seen ships with at a software package that can act as a five-function calculator, if someone is just looking for something simple.
Also, Apple gave iOS a default calculator just a few months ago. When I had an iPad I had to download dozens and dozens of calculators to find one that didnt have ads or subscriptions
Edit: I meant iPad. They sold iPads without the calculator from launch until Q4 2024
After a long time of using CalcES on my android phone and cheating the ads away with PiHole, I finally spent the very little fee for the pro edition. The app is amazing and the creator absolutely deserves it.
In a sense this is great news. If you ever need to earn some money (somehow, anyhow) you now know that there are people out they who willingly pay a subscription for their calculator.