Your point about polio is bizarre, as it was never the dominant factor in childhood mortality. The steepest decline in childhood mortality was during the period from 1850 to 1900.
You want to believe things are worse in America, I can't change your mind. In my own lifetime things have gotten visibly better. My father lived into his 90s, and he'd recount how things have gotten quite a bit better. Sure that's anecdotal, but you can look up statistics, too.
A "bunch of neat stuff" does improve standard of living. I'm almost never bored, for example.
We can easily list cool stuff that happened over the last 100 years: radio, television, cell phones, the Internet.
We can easily list bad stuff that happened over the last 100 years: environmental degradation, global warming, the increase in the percentage of income spent on transportation, difficulties in raising children.
What do we get when we subtract the bad stuff from the good stuff? Until we have good metrics for doing that, we are like the CFO who confuses gross revenue with net profit.
> what metric do you use to prove that the standard of living has gone up?
Average height, longevity, infant mortality are good proxies. Median wage doesn't mean much as it doesn't say what you can buy with it.
Have you ever been to Disneyland? Yellowstone? Traveled internationally? Only the rich could do that in 1900. People didn't even brush their teeth until after WW1. Few my age would have any teeth.
No birth control, no living together before marriage. Would you be happier with that?
No radio, no TV, no Hackernews, no movies, no stereo, no Beatles, no disco (!), are you sure you want all that?