1: Microsoft will ensure that capable hardware (both phones and peripherals) exists so that Continuum succeeds.
2: Ubuntu will leverage the hardware so that it's usable by the 1% who actually want it. 1% of a really big number is still a big number, so it'll be viable, if only barely.
First Microsoft is going to fail. They already are. Eventually they'll stop producing hardware, but I'll just assume you mean Samsung or whatever.
The hardware is specific to what each phone provides (which changes per device), and usually the hardware vendor ships the drivers closed source integrated with the rest of the OS. In the PC land we had a neutral Microsoft that packaged drivers and distributed them since anyone might want to change their video card. That's not what happens with phones. So Ubuntu will most likely need a very specific phone just as they are currently doing and selling. Which means random people can't just try it out, since the phone they already have and like isn't compatible (let alone "dual boot.")
Now Ubuntu and their partners have stock that needs to get sold, and everything unsold is a loss. I can't install their OS on my iPhone, so there's no community support to be building this in an open and decentralized way. My galaxy won't be supported well because of driver issues, and since android is already open source enough there's no big momentum there to change things either.
Microsoft with all their resources and power can't manage to keep their marketshare and is around 1.7%. 1% of smartphones is huge -- you think Microsoft's billions of dollars and a known name couldn't do it, but Ubuntu can on their own hardware that you now have to buy?
1: Microsoft will ensure that capable hardware (both phones and peripherals) exists so that Continuum succeeds.
2: Ubuntu will leverage the hardware so that it's usable by the 1% who actually want it. 1% of a really big number is still a big number, so it'll be viable, if only barely.