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.

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.

They have used a chimera though, UXKit: https://9to5mac.com/2015/02/05/uxkit-framework/