If I buy Tweetbot today, and it works today, it should keep working tomorrow. If I want new features I will pay for those features, update the app or make them an in app purchase, or create a new version number and charge for it.
This is like leasing vs buying, the only difference is with your example you'll get one choice, pay the lease, or pound dirt. I'd rather buy the app once and be done, and have the choice to upgrade at my discretion, than be forced to pay yearly.
Tweetbot is kind of an edge case since they depend on a third party. A better example would be an application that does something, like a notes app, like noteability.
Every iOS app depends on third parties: Apple, since that is the platform the app runs on; any libraries or development platforms built into the app; if the app talks to a server, the server is a dependency, and of course all the dependencies the server has...
Somewhere along these lines, the software will need to be updated repeatedly: security updates, API changes, SDK changes, etc.
Software just does not stand still like we think it does from the consumer perspective. Any app in production is going to have maintenance costs and forced upgrades that will be an ongoing cost to the developer.
I grew up in a world where software came on a disc, I bought it and installed it it was mine. A perpetual subscription model feels to me like ransomware--"pay up or you'll never see your app again." But I have to admit that I know that the costs of having an app in production today are continuous.
This is like leasing vs buying, the only difference is with your example you'll get one choice, pay the lease, or pound dirt. I'd rather buy the app once and be done, and have the choice to upgrade at my discretion, than be forced to pay yearly.