Could it be possible (as in strictly speaking, no evidence going against this) that light could be limiting towards a more fundamental speed, and that light truly does slow down when a mass goes a speed along with it, but that the difference in speed is so minuscule that it is near impossible to notice with instruments used today?
By the way, I do not consider this as a discussion, but rather a question with the answer of "possibly" or straight up "no", and evidence or reasoning supporting your claim. I will only choose the answer with the best supporting evidence or reasoning, not the "maybe" answer just because it makes me feel good.