The core OS is still a desktop one.
It would want to be... and MS would want to make it easily accessible... if not the default UI.
DisturbedComputer that was already possible in the developer beta with a simple registry tweak.
The thing with 'simple registry tweaks', is that most users are not familiar with the registry, much less going in there and changing things, so I hope you're right, in that MS will make the UI's easily [like the click of a button] interchangeable. If not, Win 8 will be the next Win ME.... won't say Vista because I found it to be a great OS [especially in x64] and never had any major issues with it.
The problem MS will face is that people as a whole don't like change, and if their first experience/sighting of Win 8 is the Metro GUI, then it will lose ground, even on Vista... and we all know the consumer uptake on that one was poor, to say the least. Despite Win 7 having a better migration to it, I still know people who are clinging onto XP like it was the last OS... EVER. And no, it'd not always about the money.
Most XP hanger-on'ers I have spoken with say it's because Vista and Win 7 offer them nothing that they can't do on XP, and thus, there is no need for a change they're not comfortable with. Shoot, I even know a bloke who still clings to '98!!!! Yup, that's right, '98! He never liked XP [feared the change, I reckon], and despite my repeated warnings regarding security, he still persists to run '98 to this day, so there's no way on Earth he and others who share his thinking will migrate to 8.