I was talking about servers, not client apps.

Regarding the app, yes, $20 is too expensive for just turning off ads. Moon Reader Pro asks $8 for an ad-free experience, and yet I think it's a bit too much for me. But it's a market, and demand will correct the price. If nobody pays $20 to disable ads, the devs can consider reducing the price, at least temporarily. So I don't see any problems here.

As for totally free apps, I consider them as a sort of gift. Some people are giving away the results of their labor for free, maybe because it's their hobby, or because of ideology, but definitely because they have spare time to work on their apps. But it can change, and active development can stop. The only thing able to motivate them to continue the work is a profit, allowing them to spend some time without sacrificing anything else. We can end up with ads, subscriptions, single-time payments, or maybe just donations. I think it's inevitable for active projects.