it's totally fair to charge for work you've done. the fact it's simple is irrelevant. what matters is the value it brings to the user.
I think it's really refreshing to find an app that doesn't lock any features behind a paywall or makes using it more cumbersome unless you pay. I'm mostly okay with one-time payments though.
Just because you invested some time into making a project doesn't mean that you absolutely need to make some money to make it "worth" it. Hell, most open-source software is built on free/voluntary labor.
I agree, and I make many projects for fun and find it rewarding when others use what I've built. But that is a decision that I make myself, for my own work. I never feel like I have the right to tell others whether they should build something with profit in mind or not.
I have been totally burned out by having to maintain all my free apps in the Play Store though, lately. Even a simple non-internet-using app needs an update every year and needs to comply with new bullshit policies every few months. It has totally changed my opinion on free vs paid apps. I still despise subscription models, but I absolutely understand that there's just no free apps out there anymore. It just costs too much of my time to keep doing it for free.
You didn't say this earlier. You said this app doesn't need to be developed with profit in mind.
Personally I would much prefer that developers lock poweruser features behind a paywall rather than plaster ugly ads all over the place. Making it a paid app works too, but likely 95% of the potential userbase would not try the app if they had to reach for their wallets first.
(I would leave the comment at that, but it would probably come across as a bit facetious and would fail any 'low-effort' test. But I genuinely mean it: remove the necessity to obtain a certain amount of money every month, and all of a sudden, people would be able to create, share, and enjoy for free.
As a developer, I feel more than sufficiently compensated by seeing people use and enjoy my work and thanking me. Getting featured on Hacker News would make my day; nay, year.
I just need to be able to eat and use a computer. I shouldn't have to prove myself valuable just to be allowed to live. I think everybody, regardless of what they do, deserve a livable basic income.