Why would they rewrite Final Cut Pro in SwiftUI? Rewriting code is a large effort and a waste of time without proper justification – new developer tooling for UI is not usually a good enough justification to rewrite the entire UI
It's not about Final Cut Pro specifically. It could be a new app. It's about having a single showcase complex app that says to developers—look hey we can build something really performance intensive and sophisticated in SwiftUI. We're dogfooding it. Look what can be accomplished with it!
It could be anything—Final Cut Pro, Logic, Pages, Numbers, a new productivity app all together. Some showcase sophisticated, complex productivity app to show what can be done with it and we're in the trenches with you using it.
The Shortcuts app in macOS 12 and the Settings app in macOS 13 are almost entirely SwiftUI and the most substantial projects in this regard
https://daringfireball.net/linked/2022/08/15/ventura-system-...