Would Apple have sold more iPhone 5's than 5c's if they just kept the 5 at $99? I doubt it - those that wanted the 5 bought one. With the 5c being the "new" iPhone and having color options gives it more appeal than the year old phone in black or white.
But you could have said that about the last two launches. In fact, Apple said that it's older models were selling better with the price reduction than in their first year of release. The 5 at $99 might have had similar results. We'll find out soon enough.