As mentioned above, Steam already does this. So...

But at least they won't charge me a monthly fee for the privilege to.

xbox doesn't charge you a monthly fee to play offline games

You're really reaching here. "Well, there's this one very specific kind of title where the Xbox is almost as good as Steam, so clearly the Xbox and Steam are roughly on the same level."

Granted - but (at least in my case, and I would surmise in the case of many - if not most - users) the majority of the use of a console is in online playing.