I am often skeptical of these claims that we will see the next generation have a shorter life expectancy as these claims require models. These models may be incorrect for a variety of reasons: misspecification, noise, shifting patterns of disease, unexpected technological improvements, and so forth.
But what was fascinating was the map in the article. The places in the United States where life expectancy is dropping are focused mainly in the Southeast. Now that distribution is, itself, interesting as the southeast has long had health issues: think of the classic stroke belt. Furthermore, it is an area of high inequality that has a climate that is very compatible with a sedentary lifestyle.
Contrast this with the California coast (and especially Los Angeles) where life expectancies are actually rising, or even New York city. Could it be that an urban lifestyle is actually life enhancing (both in terms of quantity and quality)?
So perhaps, instead, what we have is an ecological experiment to really try and understand these phenomena.