I agree codecademy was the most fun I've ever had learning a language. I'm going to college now, trying to struggle through at least one language, but the professor only reads out of the book. :/
What if they set it up in such a way where you could either upgrade your subscription or pay a one time fee and have it enabled for your account? Maybe give a trial for new users to see if try want the feature? That might take care of the "5% of users" and maybe bring in some new revenue.
I'm not quite sure if it'd work how I imagine but, if it could work, it sounds like a pretty good idea.