You are right, the laws of aerodynamics don't suddenly change at 25mph, but you are wrong that the aero advantage is the same at all speeds. The higher the speed, the higher percentage of power output goes into losses from pushing air around. Therefore the advantage is greater at higher speeds.
That really depends on how you measure the advantage now doesn't it? You chose to measure it in terms of reduced absolute power. What if you had chosen to measure the advantage in terms or percentage reduction in aero drag or reduced time to cover a set distance? Run the numbers for yourself and you'll see that in the first case, the benefit is independent of speed and in the second, the benefit goes down as speed increases.