Apple has never described Swift, Catalyst, or SwiftUI as a transition from or to anything. Instead, Apple has gone out of its way to say that these are not transitions. Remember that big "NO" slide from last year?
When a transition is required, Apple does not ever hide it.
Classic Mac to OS X was a transition.
PPC to Intel was a transition.
UIWebView to WKWebView is a transition.
SwiftUI is no more a transition from AppKit/UIKit than Storyboards were a transition from NIBs and programmatic UI.
Modern ObjC syntax, Enterprise Objects, Java/Cocoa Bridge, Ruby/Cocoa, Python/Cocoa, Key-Value Observing/Encoding, ObjC Garbage Collection, ARC, Storyboards, Playgrounds, Swift, SwiftUI, etc. These are not transitions. These are all just tools in the toolbox. Some more useful than others.
The only thing Apple's engineers are trying to do here is create stuff that developers find useful. There is no hidden or invisible strategy with this stuff.