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 Quickbooks and 90s-era video games), and connects with commodity connectors, then the idea stands a chance.
(IMHO, Windows Continuum died because of a chicken-and-egg problem -- no iOS devs wanted to pay $1000 for Visual Studio plus a Windows laptop, but no users used the phone because key apps were missing.)