That's just too niche of a use case IMHO. Samsung is doing it anyway but I'm yet to see anyone using it.
Companies seem to prefer thin client solutions where the employees simply connect to the office workstations. That way, they don't have to provide expensive light and powerful computers, anything that can connect to the internet and stream the image of the desktop does the job and they don't have to deal with the security implications of having to manage a device which is outside of the company network.
Sure, the iPhone can act as the think client but you still need to provide the screen, keyboard and mouse so maybe just slap a cheap computer to those and don't bother with providing company phone which would definitely be more expensive.
So the use case boils down to people that for some reason have access to a screen, keyboard and mouse in many places but don't have computers at these places so they provide their phones as computers. Alternatively, maybe people who would like to save money by having one computer for everything is also a target audience but AFAIK Apple doesn't go after people who want to save money.