That's not quite what's on offer. It's more like the ability to plug your phone into a monitor and carry on working. No longer carrying a laptop around (once tablets/phones are at CPU horsepower parity for office work).
So yeah, it's not working backwards like you suggest, it's making the phone/tablet more like a desktop (add mice, keyboards, screens etc.) One mobile device is quite likely all we'll need in the near future, not three separates.
2-in-1 laptops are already selling quite well as MS gets its own convergence efforts underway. Apple are actually lagging in this field and kudos to Ubuntu for steering the ship in an innovative/inevitable direction.
Do I really need a phone to do this ? As long as I can reach the same data I don't really care if I'm on the same device where I started working on it.
In fact the software I want to use when I am in front of a monitor is much different than the one I have on my phone, and I am not talking only about user interface, but also features.
Cpu power is getting cheaper and in smaller form factors. I suspect that instead of connecting our phones to the display devices, those devices will be powerful enough that we won't need any integration. And if I really want something portable, I will be able to "add a cpu" with an external stick (like Intel Compute Stick, but cheaper e/o smaller).
I'd say it's even more likely that anything computationally expensive won't be done in the same room with you to begin with 10 years from now, but will be done by an anonymous compute node in a datacenter that you're leasing for the 5 seconds you're using it.
It also doesn't work well with tasks involving non-trivial datasets. If you've got say 20GB of video from a Gopro you want to transcode, shipping it out to a datacenter is going to be really painful.
They do? I've disabled the touchpad on my surface pro 4 (and have been known to toggle it and the nub on my thinkpad) - but the touchscreen actually works great, I think. So much so, that I'm now touching all screens everywhere, like a three year old that's been given a tablet as her first computer.
I've actually thought that a good touch screen and perhaps stylus, would be the perfect environment for something like the ACME editor: https://www.youtube.com/watch?v=dP1xVpMPn8M as a new power-user editor.
It's been tried over and over again for a decade now, and keeps running very hard onto the rocks of "users really, really don't want that".
I get why devs want it: it's a single codebase for everything. But nobody, and I mean nobody, wants to use a desktop computer the way they use a tablet or a phone. There's a reason people tend to disable computer touchscreens.