You can use life expectancy as a proxy for the level of living standards. Spoiler: not everywhere

I don't see why I'd use that particular metric of living standards. There's few places in the world where I think life on average isn't better now than it was 50 years ago, even if you ignore innovations like the internet.

The things killing people in the developed world are diseases of excess. People literally have too much stuff.

It's also possible they're diseases of despair. Advanced economies are highly competitive, and if you are losing in that competition it's crushing, even if your standard of living is good in an absolute sense.