Now, I can see problems too: docked and portable modes need very different performance optimizations. But I'm sure that software can handle this, for example, IDEA IntelliJ has power save mode, and OS-es also demonstrated that they are fine on portable and connected systems alike, like MacOS, Windows, Linux.
It's also not a problem that some things are not available in both modes. For example, Switch has games that explicitly need docked mode, for example, Super Mario Party. Yet both the game, and the platform is popular.
I see no reason why a phone couldn't be a mediocre, or better PC.
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.