The problem with solutions like Flutter is that many apps, especially iOS apps (though maybe not a calendar app), will often use powerful platform features on i

The problem with solutions like Flutter is that many apps, especially iOS apps (though maybe not a calendar app), will often use powerful platform features on iOS that are not so readily available on other platforms, so some degree of platform-specific code is always going to be there -- maybe a lot of such code. For example, the amazing Core Image library, or writing Metal kernels (increasingly common since it is rather easy to leverage GPU compute on iOS and offers tremendous performance benefits), and other tools. If you are going to interact with that in Swift anyway, adding a new language and UI framework and extra layer of abstraction and communication to your app may not make sense.