Cocoa over the last few years has made a lot of cleanup/progress on its api's. Making a Mac app in Swift 3 is almost as painless as making an iOS app nowadays. I feel like there has been a big difference in just the last few years. Swift is a big part of this, but also, I'm sure Federighi's group is the real reason.
This is one of the big reasons why I don't buy the whole narrative in these parts around Apple abandoning the Mac. They are spending far too much time on improving Cocoa APIs as you mention to be working towards abandoning or deprecating it.
I'm not familiar with either API, but why wouldn't it be possible to get Cocoa working in iOS? I imagine UIKit would be optimized for small, low power, devices, which will always exist in some, ever shrinking, form.
It's all just a BSD variant, when you get down to it.
It depends on app and developer tool support. Raskell (Haskell IDE for iOS) and Pythonista (Python IDE for iOS) are fun enough to use. While I would really like IntelliJ and RubyMine on my iPad Pro, I really like macOS.
All this said, I like to see completion. There are some very interesting Window 10 hybrid devices, and the new tiltable desktop Surface look interesting. I hope that there is never a 'winner take all' for consumer devices.