Declining life expectancy is not an indication that the population is poor either. Consider that obesity, which is linked to higher mortality rates, did not become an issue until the 1900s in countries that passed wealth thresholds allowing such a problem at scale. Similarly with allergies, car crash fatalities, lead poisoning in paint, any number of life-shortening effects come from wealth in a society and are counterbalanced by improving health care levels.
Given how small the US decline is, were China to stop the massive supplies of fentanyl they're intentionally allowing to pour into the US, that decline would probably arrest. The Chinese fentanyl is killing people at a young age, which hammers the life expectancy calculations.
Remind me how China feels about what was done to them in the Opium Wars.
The US isn't alone in this however. Britain is starting to see declining life expectancies. Is China better off than Britain? It's laughable.
"UK life expectancy drops while other western countries improve"
https://www.nhs.uk/news/medical-practice/uk-life-expectancy-...
https://www.theguardian.com/society/2019/mar/07/life-expecta...