I have traveled with just my iPhone and can get by but don't really love it.
- https://support.apple.com/guide/keynote-iphone/present-on-a-...
I'm skeptical that there's "no demand" for that kind of functionality rather than a lack of good implementations. Look at how popular wireless CarPlay and Android Auto are. They're essentially the same functionality, but tailored to an in-car experience instead of desktop.
The first flaw in the idea is that computing is cheap. You can make a computer the size of a phone for people to carry around, that has been tried but failed. The second flaw is that everything is in the cloud, only developers and offline need local access to their files. The cloud also means that can desktop in the cloud.
If Apple releases a $300 lapdock tomorrow, basically a screen, keyboard, battery, that allows using your iPhone as a normal general purpose computer with OSX - why would anyone buy a laptop/desktop?
If you are doing serious work, which are the people who want a dock, then you need the power of Macbook Air or Macbook Pro.
For most people, iPad or iPad Air with keyboard is a better option since you get tablet for fun and can do some light work.