It depends on your reference point, but IMHO there's no device right now that hits all the right point, so yes, Surface Pro is one of these flawwd machines.
On the other side you'll have devices that feel really well built and graceful, but can actually do very little, or other ones fitting a very average vision of what a computer needs to do, and you'll be paying for additional devices to deal with the edge cases.
The real big roadblock is Apple, but if the DMA forces them to let third party software, we could get a fully exposed subsystem opening the door to what users really ask for.
Right now the joke is Windows XP emulation making it what it always needed to be, getting containerised/emulated Mac apps with decent Perfs from low level access would be a huge win. We could be close to your ideal, with the iPad still running, and a Mac instance pinned to the external screen.
This is my ideal setup. And I'd have it switch to macOS mode just with keyboard/mouse, so inside the magic keyboard it is just the most slick 11" macbook air ever built. Pop it out and you are dropped back into iOS.
I'd easily pay $3k for a top end version of such a device. I think this is Apple's main holdup - if the iPad can run macOS in this dual mode setup, the MacBook Air becomes pretty boring and a pretty bad deal. And they can no longer sell people two devices that accomplish the same task, only differentiated by one having a touchscreen.
In my eyes, Apple's transition to ARM on Macbooks looks like a stepping stone on that path. I wouldn't be surprised if they announced something like that for the iPad Pro eventually.
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.
On the other side you'll have devices that feel really well built and graceful, but can actually do very little, or other ones fitting a very average vision of what a computer needs to do, and you'll be paying for additional devices to deal with the edge cases.