The "one device, multiple contexts" thing I think rises above the tinkerer niche. But only if Canonical does it right.
Here's what I think would need to happen for Canonical to reach mainstream success:
1. They'd have to ship a powerful smartphone that can transform into a tablet or a laptop using a shell peripheral, as well as support a desktop experience using an external keyboard, mouse, and monitor. That way one device can be your smartphone, tablet, laptop, and desktop all at once.
2. It would have to be an awesome user experience in all four contexts. All apps would have to have responsive designs capable of adapting to the context transforming while still dealing with the same user data.
3. OS updates must continue to work as they currently do in Ubuntu. I get them from Canonical. Cell phone carriers should not be allowed to be involved in the process for the same reason my ISP does not decide what updates I install on my desktop or laptop.
4. Apple, Microsoft, Google, etc have to not beat Canonical to it. MS already has the Surface product which is teetering in that direction, but isn't quite there yet. So we know the big players are interested.
What worries me is I think there's a good chance that Apple, Microsoft, or Google will deliver #1 and #2 first, which will kill Canonical's chances. But if miraculously Canonical did it first, I trust them to deliver #3. I don't trust their competitors to deliver #3. Least of all Google, sadly.