Subscriptions are fine as long as the customer feels like they are getting something for it or they realize there is a constant cost to the developer that needs to be covered. For example, if your app relies on a server, someone is going to have to pay for the server, and while some developers eat the cost themselves, they shouldn’t be expected to. What people don’t want, is to pay for a subscription that offers nothing other than the continual use of features you already paid for. There are quite a few mastodon clients that do this and it pisses me off.