FWIW, the Apple developers discussing SwiftUI on Twitter are careful to point out that SwiftUI is not designed to be a wrapper around UIKit. Much of the implementation currently uses UIKit under the hood, but that's an implementation detail that is subject to change and there are already significant parts of SwiftUI that don't use it.

It seems as though they are viewing SwiftUI as a successor to UIKit/AppKit on all their platforms, not just a higher level wrapper.