My question has always been - will Khan Academy lose relevance outside Sal Khan? He can't be an expert in everything. In order to scale, I'd expect that he'd have to be able to pick people who are great teachers and also domain experts.
The potential for misinformation from someone who is teaching a subject that they are not an expert in, and have just "understood it quickly" is quite worrying. he has been criticised on this before, see http://www.nas.org/polArticles.cfm?Doc_Id=2029
Economics is a lot like maths, and there is crossover so it's no surprise that he is good at both. But History is a different kind of subject.
If he is really irreplaceable, then there are severe limits on it. I hope and expect that even if Sal Khan is a trailblazer, others can do follow his example.
But let's post a link to Khan's comments/rebuttals to the claims of the NAS article you cited.
Get non-domain expert, smart teachers to study and try to teach a subject ?