Agreed, i hacked together something resembling a midi-sequencer UI in about 1000loc with JS+Vue.
I have an old editor written in Java/Swing that takes more and more willpower to work on every time because handling state update bugs is what i spend half my time on.
It is the fully working safari UI in only 1000 lines of code. Additionally the declarative approach of SwiftUI together with the type system makes it certainly correct in many ways: no rendering bugs, no invalid states, probably no memory leaks, no cyclic dependencies creeping in.
Try to build this in Java Swing, UIKit, Html/Css/Js or any other UI library. Flutter might come close, react/vue/svelte as well, if you assume sensible predefined components. But they work similar to SwiftUI and the resulting one-file might look pretty similar.