Which games take off is not super correlated with quality; I would argue that Angry Birds is above average, but not exceptional. The whole "hit" phenomenon is highly dependent on initial conditions, a sort of semi-random positive feedback loop. When building a game, one's idea of success should be the cultivation of a dedicated audience large enough to sustain your business. Counting phantom billions will lead to terrible design decisions, usually imitating random aspects of successful projects, like some statistically-illiterate lotto player diligently searching for patterns in previous winning numbers.