I assumed Metro scaled well, regardless of screen size.
Even before Metro was made available to the public, I remember a lot of write ups from Microsoft employees stating that both Metro AND the Desktop were here to stay. Metro is not and apparently was never intended to be a Desktop replacement. Metro's primary focus is touch supported media consumption. In contrast, the Desktop is about high density presentation and flexibility. One can't replace the other, simply because they are optimized for very different and conflicting usage scenarios.
Windows has long grappled with the impossible task of trying to be all things to all people. IMHO this is one of the reasons Windows has gained a poor reputation. It's simply impossible to make an OS that can be appreciated by both an engineer, and the majority of technically illiterate consumers. Metro is MS' attempt to offer a separate UI specifically for the later group of computer users.
Don't get me wrong. I'm not saying Metro is only for the technically ignorant. Metro is for everyone. The difference to years past is that MS no longer considers the Desktop to be for everyone. That should also provide MS the flexibility to offer Desktop features that it previously couldn't. I remember reading an article somewhere (don't know if legitimate and I don't have a link), where it was stated that supporting multiple desktops was one such scenario, where usability testing showed that it wasn't a viable solution for the average consumer, although computer professionals everywhere would love it. That's just one example. I can think of many more. I don't know if this is what MS is actually planning. Based on everything I've read, I expect it is.