Eh, fidelity to the source material is just one factor. Prior to Marvel adaptations and Batman Begins, a lot of the films were just based on or using characters (or the ideas behind the characters), filmmakers putting their own spin on the characters not the actual comic book. The first 3 Batman films did well despite pretty much being their own thing, Daredevil was made by a huge fan, replicating some material, but didn't do fantastically, and its debatable how close the X-Men movies were-arguably a source of their strength was that it made the characters and look accessible and realistic rather than too complex or fantastic.
I'd say that the Potter movies, aside from the third, were faithful, in fact too overloaded, and they were still successful.
I wonder, though, can you be faithful to the source material but suck as a film, and still be successful?
I'd say that the Potter movies, aside from the third, were faithful, in fact too overloaded, and they were still successful.