G&T
An aircraft flies due to the difference between the speed of the aircraft and the speed of the wind.
The simplest and most reliable method of doing this is a pitot.
A perfect inertial nav system (which doesn't exist, incidentally) would still be useless if it had no way of knowing the speed of the air around it.
ie, the air would also have to be dotted with inertial nav systems in constant radio comunication with the aircrafts inertial nav system to make this an effective method of working out airspeed.