True enough, bookworm, and that is why windshear is an issue, though whether it is the air or the aircraft that is changing velocity is ALSO a matter of what frame of reference you are using.
But momentum is not conserved in non-inertial frames. Thus the frame of the air is about as much use as a chocolate teapot when it comes to working out why your aircraft is sinking into the weeds because you failed to take account of the 30 knot drop in headwind in planning your approach! Minimum groundspeed looks very sensible to me.
The misconception, however, that velocity is mesurable only reference the earths surface is what leads to faulty concepts like the infamous "downwind turn" myth.
I don't agree with that. It's perfectly possible to debunk "the infamous downwind turn myth" in any inertial frame -- you just have to remember that velocity, and therefore momentum, is a vector not a scalar.