A professor standing in front of a group of students lecturing is definitely easy for the professor and cost-efficient to scale up to larger class sizes, but that's just not how a lot of students learn. I'd wager that most HNers learned programming through actually writing code, even those who learned CS through a formal program. In the humanities, I'd argue that the most effective way to learn is through small discussion groups, not a distinguished professor explaining literature or philosophical works to you. Just throwing that up on the internet is an easy way to expand your audience, but providing higher-quality educational materials doesn't do anything to improve the quality of how we educate.
The internet has a lot of potential to improve the quality of education, and there are tons of awesome startups working on it (companies like Codecademy and Coursekit come to mind), but I personally hope the future of online education doesn't look too much like Udacity.