I mean, you say this, but even before the TBC announcement, tons of people were still leveling characters in vanilla. I leveled a Warrior around December/January and every single zone was still decently populated. Finding dungeon groups wasn't too hard, though that's likely because I was a tank so I could find dps/healers real easily.
A once-per-account level boost isn't going to "hurt leveling" very much at all.