Not at all, in many vehicles. 6th gear in my car is quite long (Turns 1950RPM at 75MPH) and I have no problem maintaining speed on highway grades.
Even if you do downshift that doesn't necessarily mean you're being less efficient either. Being a bit higher up the rev range may actually put you in a better spot on the efficiency curve for a given power output.
But you raise good points, maybe I jumped the gun on 'physically impossible'.