I suspect at some point Office will release a bunch of apps targetted at the latter group (remember Microsoft Writer for blog-writing?), or even have a dual mode where you can write basic content into Word whilst on a train, and can then flip back to Desktop mode to finalize it. I don't think there's any chance of them moving to Metro-mode wholesale in the near future.
The suggestion that you'd be fighting excel on a tablet screen in Metro-mode sounds dreadful, but that doesn't mean that there isn't a middle ground where some parts of Office move to Metro, other parts stay in Desktop and some apps (like Outlook) sit somewhere in the middle.
My WP7 has excel. I've used it exactly once; long enough to know that it was not a useful tool in that form factor.
I feel kind of sorry for the devs that spent time working on it...