Ubuntu convergence exists, so technically its doable.
Its not as useful as you would think. You can access most commonly used apps as a web app on a laptop or a desktop. The only real advantage would be file locality, but phone storage is somewhat small.
There is a small use case of mobile gaming where you want to play a mobile game with a keyboard and mouse, but that is very niche and custom solutions exist for this.
I personally do this today with an iPad and remote desktop - but I always have a desktop running for other reasons so the marginal increase in use is acceptable.
I think what you want is an iPad but I do wonder if the USB-C iPhone 15 Pro will be able to support more of what you mentioned.
I'd love to be able to do this. But the phone would have to run MacOS — iOS is too locked down.
The only thing that could prevent me from doing this is that the phone screen is small and i'm not that tech savy, so i probably couldn't find out how to put it onto a bigger screen, but if your Tech Savy enough I think this could totally work for you!
USB-C actually unlocks this I believe.
>Is there an appetite for such a product if the app ecosystem was also available ?
No, not really. Samsung has DeX[1] which is exactly what you describe - connect your phone to a USB-C hub, plug in HDMI monitor, keyboard and mouse and you'll have phone as a computer with full windowing operating system. I tried it once with my Galaxy S21+ which would be fast enough to be my daily driver computer, but for many reasons it just didn't work right. Maybe it's Samsung's DeX but I doubt it because using an iPad as a laptop also doesn't work (for most people including me).
[1] https://www.samsung.com/us/apps/dex/