Because making the new behave like the old, while the new is not the same as the old, is itself a thing to be tested. Running the happy path is not sufficient t

Because making the new behave like the old, while the new is not the same as the old, is itself a thing to be tested. Running the happy path is not sufficient testing. Also, the test pilots literally have skin in the game. If the point of an airplane is to fly, why have test pilots?