I've always been fascinated by this idea, especially since finding Justin Blanton's blog post on the subject:
OK, that’s not entirely true — my non-work MacBook Pro is an amazing machine for backing up my pocket computer. Yes, I’m being sardonic, but not entirely. I really don’t use the MBP for much of anything these days, and frankly, I’m not sure I’ll ever
justinblanton.net
I've done a lot of experimenting with input devices, external displays, and Remote Desktop solutions to fill in the gaps. Even if the iPhone is not my only computer per se, it can be the only device I interact with directly and keep at hand.
I think the iPhone 7 Plus and 8 Plus were better for this than the X generation, since the home screen rotates and there isn't a notch getting in the way while in landscape mode. Remoting into a desktop is much better without the rounded corners. Also I don't think FaceID works when the phone is on its side.
If you can keep a Mac or PC powered on and connected to a network, then connecting an iPhone to an external display, a bluetooth keyboard, and Citrix mouse can be a very convincing substitute for a local machine, since Jump Desktop can adjust the resolution on an external display independently. It's pretty neat, but I would caution that it is fiddly.
Nowadays, I would posit that the ultimate one-device is the cellular iPad mini, provided you wear pants or coats with large enough pockets to accommodate one. I might consider a Brydge keyboard to attach to it, or maybe one of those bluetooth keyboard and trackpad combinations. This way you'd have real mouse or trackpad support, Pencil support, and iPadOS multitasking. Even better would be if they gave the mini a USB-C port for better connectivity to a display. You wouldn't have native phone functionality, but there are ways around that, and the slightly larger display is useful in a lot of ways.