Catalyst is the stopgap before SwiftUI. Eventually we'll see all Apple apps migrated to SwiftUI from AppKit, UIKit, and Catalyst.
I like SwiftUI. It shows a lot of promise, and I look forward to releasing products, written in it. I don't think it's especially mature, and maturity is important, when you need to release ambitious projects.
I'm not really in a hurry, though. I'll be getting around to working in SwiftUI, and I tend to do well at the things to which I apply myself.