Pro lines are premium until they become the norm (MBP), in which case the pro line is killed (or specialized) in favor of the default line (Macbook). 'Light' or

Pro lines are premium until they become the norm (MBP), in which case the pro line is killed (or specialized) in favor of the default line (Macbook). 'Light' or 'Air' lines are minimized initially, and then their internals come up to parity with the default line and they are killed off for being redundant. Then you have a generation or two of iteration for the main line before new Pro and Light lines (with new branding) come out with 'innovative new features' of some higher specs or lighter weight. It's like a brand model treadmill.