We're really close to that already. (I haven't seen a desktop computer under a desk in years -- basically every modern office, is a bunch of perhreprials on a dock, tied to one single USB-C/Thunderbolt cable, that gets plugged into a laptop.

If iPhones / iPads ever get macOS or equivilent, I'm pretty sure people would switch. It's mostly an Apple-enforced rule preventing it, not necessarily any actual software or hardware.