For developers this transition was quite seamless, since most of the high-level API functionality was still there. This isn't the case when you're ripping out the current UI framework that every app uses.

I am not saying that's easy but unifying development for desktop and mobile seems harder. I think a lot of people have concluded that it's pretty much impossible because the needs are too different.

That was over ten years ago. The Apple of 2017 is a totally different company, and I don't think they're capable of pulling this off.