The first version is the part everyone plans for. Then the operating system updates, a store review rejects a release, the app has to work with no signal in a warehouse, and the backend it depends on changes underneath it.
An app is a product with two stores, thousands of device combinations and users who never update. Building it like a website is how it ends up with one-star reviews six months in.
So we plan the release process, the offline behaviour and the backend contract before the first screen, and we stay on the app after launch.