I think that's eminently reasonable. As someone with 15 years experience doing Cocoa dev, the intention to be a Mac dev indefinitely, and hundreds of thousands of lines of existing Cocoa and Cocoa Touch code, it's just going to be a slow transition. But there's no pressing reason to just stop writing any new Cocoa code (I literally wrote some since my last comment :-P). If I were starting now, I'd probably focus on SwiftUI with the assumption that it will be the way to start most new projects within the next few years.