A one time cost is fine if you don’t mind the app breaking next time Apple updates iOS. There is an ongoing cost to ensuring the app continues to work.
The maintenance effort required on iOS is substantial. About a quarter of your full-time year needs to be dedicated to it.
On desktop, you can just publish your software and slowly see it age as you work on your next big release. On iOS, it ages every year at brutal pace, and your new sales will plummet while you work on your next big release, meaning your revenue crashes much faster.
Even worse, the iOS App Store has no notion of paid upgrades, and publishing a new app is basically like starting from scratch as far as discoverability goes. So when you finally have your next big release ready, it's like launching a completely new company.
Apple really wants developers to make subscription apps that ship frequent iterative changes, and other business models just simply don't work well on their mobile platform (on Android it's even worse btw).
