The problem with this idea is that it's unattainable.
Let's start with something simpler: a living room. There's no universal design that would fit just any living room. The layout and the set of furniture that would work for my living room will not work for yours. Size, shape, windows, doors, connection to other spaces - everything matters. If you want a great design for your living room, you literally need to start from your specific living room.
There's a great idea -- why don't we come with a resizable (reflowable?) design that could fit any living room in the world? While this idea might be entertaining for an engineer's mind, it doesn't work in practice, unless you can settle for just a mediocre design.
Also, being intellectually honest, we need to attack the strongest Apple we can imagine, not a weak Apple that's easy for us to attack. And that strongest Apple will never adopt this idea because they aim to design the best computer/phone/tablet that they can, and in order to design that they need to start with the computer/phone/tablet.
The idea of a phone connecting to a display/keyboard/mouse and becoming a computer has the problem that you could either optimize your design for what you have with a phone, or for what you have with a display and peripherals. It will never be as good as the system designed from the ground up to be just a single thing. It's always nice to have options, but there won't be any mass adoption for the mediocre combo. It was dead in the water with Palm Foleo in 2007, it's just as dead in the water 18 years later.