SwiftUI Requires Platform Knowledge On Top. Case Study: Fonts
SwiftUI very likely is the future of app development. But it cannot, on its own replace UIKit or AppKit. Not yet, maybe not next year, either. Eventually you will have to drop down a level to implement a custom view, custom navigation, animation, window, or what have you. So for the time being, the two worlds of UIKit and SwiftUI co-exist and complement each other.