I love Dropbox, but Jobs is right -- they are a feature, not a product. They will be relevant up to the point where operating systems simply makes their functionality a native element of their file system. At least Drew is wise to the fact that they could become an also-ran, so it's not like this will happen to them overnight.
Anybody who was a mac user before 2000 can tell you how painful it was to use a mac when your friends or coworkers had windows. It was horrible. Problems with file sharing between systems created huge network effects for microsoft. It killed the possibility of having a mac for a lot of people.
The fundamental problem of compatibility requires not only technological and architectural commitment but also willingness to cooperate with competitors for the sake of the user.