That assumption doesnt always hold true and even when true the internet is slow and unreliable and depending on it makes webapps slow and unreliable.
While this is just my guess, I honestly think there should be more users with constant internet access than all emacs users(no harsh feelings, I use emacs myself, my point is just we are a minority group).
Really the problem we should solve when using this kind of editor is spotty connection(like in an airport or at a conference), it's not a use case targeting environment without any internet access.
IMHO, atom in browser will be perfect for online code school.
That'll stop the obvious debate that always ensues...