A medium can have independent dynamics (consider waves in a bathtub: you can pull the plug, or move the whole thing around; you can't do that with a fundamental

A medium can have independent dynamics (consider waves in a bathtub: you can pull the plug, or move the whole thing around; you can't do that with a fundamental field). It is true that from a slightly different perspective, modern physics has arguably resurrected a kind of aether (think condensates in quantum chromodynamics) - it's just that said aether is Lorentz-invariant...