Earlier quoted context omitted.
I think the biggest problem is the price point. The person who most wants to use their phone as a PC is the person who can't afford spending $4000 on the entire suite of Apple devices. That person probably also can't afford a $100 dock (let alone a $300 one) for their phone. When someone makes a USB-C/Thunderbolt/Intel/Windows device that is phone sized but runs legacy Windows applications (like MS Paint and Quickboo…
I don't think the price comparison is fair. If someone spends 700$ on a phone, he would be ok spending 200$ if the system does work smoothly. What I think people are more skeptic about is performance and user experience.
People will feel that they can absorb $20 or even $30 extra per month on their plan for a top tier phone. Absorbing any hit over $200 when someone lives hand to mouth makes these a non-starter.
The best chance for mass success for this option is for it to be included in the base purchase.