a) Files - Dropbox b) Activity - Various browser sync for bookmarks, history (and passwords) c) Handoff - Mac is pretty good here. Windows has no equivalent.
You seem to have a misunderstanding of how continuum works in Windows. The interface is separate when you hook up a phone into a dock (it's a desktop interface) and the newer devices can also do it wirelessly.
Windows went with a "similar UI language" for all platforms but still maintained huge differences between each platform because that made sense. It was never the same UI.
As you said, unified interfaces aren't that important. I'd be perfectly happy with a phone that runs a Linux desktop while docked and gives me an Android GUI/Stack while in "portable mode". The only thing really needed then would be the possibility to open the Android part in a window in the "Desktop mode", so one could still do messaging etc. while docked.
Personally, I think that would already get us 80% there. The much bigger hurdle (and afaik the one Canonical had trouble with) is then establishing a market position.
Who cares if you have the same interface on your phone and PC. I mean seriously, there is absolutely no point and you end up wasting time designing some sort of interface that doesn't suck on either platform.
Incidentally, what you describe as data convergence is part of a theoretical computing model I call PAO: http://tiamat.tsotech.com/pao
This is a ton of added complexity to developers without any real return but for ubuntu.
And ties programs to ubuntu flavored linux, unless obe is willing to do even more work for free.
At least in the demos I saw, the mobile UI would turn into a traditional Ubuntu Unity look, with floating windows and keyboard+mouse interaction.
So it sounds like they were working in the direction you describe.
The dumbest move, IMO, was not using Wayland and working with that dev community to improve it's functionality on mobile (one of the reasons given for Mir). Shuttleworth seems to point this out in the post.
Turns out rolling yet another display server rankles noses and takes a lot of dev cycles.
You should be able to shift your view of a document from a desktop to a laptop, but that doesn't mean the fundamental interface from one should be shoe-horned into the other.
That could mean a phone that is docked, but when you have 10x the screen real estate, a keyboard and a mouse the interface should be different when docked vs when not docked.