To answer my own question, I think we need to see more clearly differentiated tracks for classes and more clearly defined prerequisite structures.
The online class model is a significant improvement over 'learning things on the internet with disjointed articles, videos, and wikipedia' because the duration is long enough to build up from simple first principles to more complicated ideas. Now they need to improve a step further with clearer prerequisites. Rather than teaching a quantum mechanics class where "you don't need to know calculus! We will present the material in the most accessible way possible", they can teach a quantum mechanics with a calculus prerequisite (...and here's a link to the calculus class you should take first; you can sign up without it of course, but we will explicitly assume that knowledge.)
And/or, different tracks of classes should be more clearly defined. Already many classes have 'optional' assignments and supplementary material for more advanced students, and their certificate of completion sometimes says something about that. Why not make it more explicit? Every certificate of completion should say which track out of which options it's for, what the prerequisites were, and what your score was compared to the average among people who completed all coursework.
I don't think these things are directly beneficial to the "maximize audience in the short run" objective of course offerers, in play at this early point in time. But I also don't think it's contrary to that objective, and could be done in a clearly beneficial way. It could maintain current interest but also attract additional interest and conglomerate statistics of "number of people who took any track of this class or initiated this course sequence" are just as impressive as "60,000 people signed up for this course".
But more importantly, I think these changes are vital for long-run success of online education. People won't chase meaningless certificates if they want credentials, and they won't chase empty dumbed-down curricula if they want real education and employers won't give a crap about certificates that don't have a clear meaning. It's great for those who want a cursory introduction, for fun or curiosity or a jumping off point for more serious independent learning, but that's nothing that's going to ever be able to compete or seriously supplement traditional education. It's something worthwhile in its own right, but it's not "online education".