Well, off the top of my head I know two independent developers who are making a go of it and have since basically the App Store started. Marco Arment (Instapaper and now Overcast) and David Smith (at least a dozen apps).

Software has zero marginal cost. You either have to a product that incentivizes people to pay more or enough paying customers to be sustainable. Independent developers can do the same scammy techniques that the big boys use - go after whales or advertising.

Or the less scammy free with ads with an in app payment to get rid of ads and optional in app purchases that aren’t pay to win (like Crossy Roads).