Oh sure, when we are talking about orbital paths of planets and moons and the sun, relative motion can be said to be valid (and this is completely discounting any aether). But what about when we are talking about motion within a medium such as air? There is obviously a difference the two mediums, space and the air, is there not? How would this affect the calculation of relative motion? Well, the "original motion" component would not be changed, true, but the drag effect sure would. And what would be the "carrying force" working to maintain the original motion?
Let's use my favorite airplane flight example to crunch this "original motion" factor. Yes, a plane could be said to be rotating at the 600 mph speed of O'Hare airport in Chicago when it departed. And then, as the plane left the surface of the ground, it separated from what was its obvious "engine" or "original motion carrying force" and was definitely running head on into major drag, the air. Contrasted with the example of a rocket having blasted off the earth and heading to the moon, the rocket will retain the original speed of the earth that it was on because the rocket is flying to the moon in a vacuum (for the sake of this argument anyway), and there is no drag to slow down its original motion.
But what keeps the airplane's "original motion" going? One can only postulate so many things: gravity or the air being the two obvious ones. But gravity only pulls downward and not laterally, and the air, well, it cannot have any ability to "push" or "carry" the plane because it is a gas. (Remember, if it's a rotating-with-the-earth solid, the plane couldn't fly through it anyway...CATCH 22)
The natural effect of the plane not having an impetus to maintain its "original motion" is that the plane will obviously lose angular velocity to the earth's faster rotation below. And if it loses even a little, will it not lose it entirely? Of course. Whatever link there was between the two has been severed. The drag of the atmosphere will illustrate this every time.
What this all means is that if the "original motion" component of the plane is not maintained, then explaining its observed travel reality by resorting to "relative motion" (just like for outer space bodies where the original motion IS maintained) is fatally wounded. So therefore, to conclude that the difference in mediums does not affect the way relative motion would work is not logical, but that is the enemy's stance, as far as I can tell.
It is this fact of the nature of the air is what makes my airplane hypothesis stand proud and tall. Although the airplanes would not prove anything if they were flying around the earth in a vacuum (as if that were possible!), they prove quite a lot by flying in the air. After all the dissecting of the relative motion argument, I can yet show that the planes still have the same conundrum of east/west travel that I first thought they did when I just used common sense.