You're also ignoring rolling resistance, weight will impact that.Correct! I never said how fast you get to 80mph. But once there, the wight is already moving at 80, so weight not relevant.
EDIT: Well... technically, there is more weight on the wheel bearings so there is a teensy bit more resistance on the heavier vehicle. But hats off to well greased modern wheel bearings!
Overall your point is right though. When speaking about power the aero has a factor of v^3 but the rolling resistance and drag only are proportional to v so the faster you go the more correct you are.