I disagree completely. In 2018, we constantly saw the trailing driver choose to follow directly in the tire tracks of the leading car - in the dirtiest of dirty air - rather than picking a different line. We saw this even on the multi-groove tracks. Then once he closed right up, watch the leading car get loose and wash up the hill, or watch the trailing car dart low to pass. Seeing that happen over and over tells me dirty air in 2018 was not nearly the problem it is in 2019.
In the low downforce era, blaming "dirty air" was a remnant of a lazy habit... anything one doesn't like just blame it on dirty air. Journalists, social media posts, even drivers overused that crutch, IMO. The actual fact is... there were many, many examples of cars slicing through the field from the back to the front, passing others under green flag conditions to get there. Yes, it was the good old days of racing.