In a complex, mature system, a high impact bug could have a very small fix that is highly non-obvious. Your metric assumes that the person shitting out 1000 lines of a new feature no one wants is equally as productive as a distributed systems wizard who can fix bugs no one else can figure out adding a 3 line fix for an issue that customers have been complaining about for years. It is inherently biased towards adding new features and against maintenance and system quality improvement.