So iDEX? There have been multiple attempts at that from motorola, the nokia n900, sailfish, ubuntu touch, linux on DEX, DEX, maruOS, windows whatever, citrix,...
Sounds nice in theory but people rarely actually use it.
It actually works very well. Phones/tablets are now more capable than many PCs/Macs. When you've literally got more compute power, RAM, storage, and network bandwidth than supercomputer centers had 15-20 years ago in a phone or tablet-sized package, all you really need is a nice dock to plug it into for display (I'll take a 42" multitouch/pen setup like the Surface Studio, please), keyboard, mouse, and network.
BTW, I've done exactly this daily with the only slightly larger Surface Pros and docks for over a decade, so the concept definitely works, and there are probably millions of people using it, contrary to your assertion.
It's a very small step from doing that with a PC or tablet to doing that with a folding phone design, and there are a few such solutions like that today. (Though they should run the same OS/interface, just morphed slightly for the hardware that's active.)
After having this setup, I will never, ever, go back to an old caveman laptop or desktop computer.
Sounds nice in theory but people rarely actually use it.