Are storyboard and UIKit due to disappear ?

Good question. You are far from the only one who has this concern. Many developers prefer the "old way" of making apps and find SwiftUI to be limiting, hidden behind a wall of "magic" Apple controls. We have to inherit all their bugs. In AppKit/UIKit we do too but we have a much greater chance at implementing our own workarounds.

I think most developers, even the ones who like SwiftUI at least want the option to be able to drop into the lower-level APIs SwiftUI is wrapping. For example in most cases on iOS SwiftUI seems to rely on UIKit like SwiftData is built on top of CoreData (I could be wrong on some of those details, anyone out there feel free to fact check me if I am).

This doesn't appear to be the direction Apple is heading in (giving developers the choice). Feels like Apple is trying to force this on us instead of letting the framework speak for itself. I'm getting tired of the way they are bullying developers. They are flexing their power a little too much for my liking. They want to treat us like babies. Don't you worry your little head about what's going on under the covers. Here's SwiftUI peasant...you can use that... what's underneath it...well that's not for you.

I also understand that Xcode is a very complex piece of code and that maintaining and evolving some tools (as IBDesignable) requires effort

I don't think they've touched IBDesignables in years (could be wrong). It's been frozen in time, for a long time. Good thing about that is you don't get new bugs when they stop touching things.