Now, look at the historical record of the velocity of light measurements over the few hundred years it's been done. It's slowing down. Calculations for the speed of light over the years have been done in different ways. If the same methods were employed over the years and the results showed a decrease, you might be able to make this argument. Not saying it isn't true, but I haven't seen the data to support it.
Still, if we accept your premise as true that light is slowing down, don't we then have to explain how that is happening? The concept of light slowing down throughout the universe is no easier for me to explain than the idea that the expansion of the universe is accellerating.
I agree, the experiments are all different and they don't measure the one way velocity of light in a true inertial frame. The same thing that casts doubt on the universe expanding, light slowing down also throws its shadow upon c being a universal constant. Some physicists do make the claim that c is constant locally.
Here's a try for the explanation.
First:
http://www.mathpages.com/rr/s6-01/6-01.htm
Shows that lightspeed is a function of gravitational potential by conventionally accepted physics.
Secondly, if one accepts the premise that the universe is expanding (conventionally accepted physics), and mass in conserved, then the "average gravitational potential" in the universe must be decreasing (it can increase in some regions too).
If the potential decreases, then the speed of light should decrease too. (This is observed by gravitational redshift).
Neolithic man had his shamans and witch doctors, the medieval times had wizards and alchemists. They're still here today, as they always were, feeding at the teat of those in power, claiming special knowledge (with internal contradictions they dare not let be known) that few could really understand.
"Surely as water will wet us,
Surely as fire will burn,
The Gods of the copybook headings
With terror and slaughter return"