It would be more fair to compare a phone that has desktop features, to a phone that doesn't have desktop features.
So let's compare the best Apple phone that refuses to have a dex like experience; to a Samsung that has had a dex experience for about 10 years, or to a Google phone that is now adopting desktop experiences.
If the future is anything like the past, in 5 to 10 years from now we'll see a desktop experience on iPhone and they're going to be snobby about it.
In another words, I think that the functionality itself is very useful. It's just that it's currently being served somewhat adequately by cloud based solutions. For this reason, such a phone-pc product could not offer much in terms of functionality, and so, it might not be popular at all.