I agree that not every app must be cross-platform. Text editors are a good example. The economics may or may not be more difficult. Yes the audience is smaller but the cost may be lower as well and you may have a competitive edge over cross-platform apps.

The problem with starting a single platform app is that you have to decide very early on that you're not going to need any collaboration/sharing features (unless the app is built on top of some cross-platform protocol or file format) and that you're not going to serve multi-platform users any time soon. This is a big decision with far reaching consequences.

I think there's a risk for developers (and tech bloggers) to become so enamoured with the fine details of their platform of choice that they lose touch with the priorities of the user community.