Quite the contrary. Medical history is actually something of a hobby of my wife's, and as a result we have quite a little library dedicated to that end. In addition to that, there are many occasions where I politely listen to her when she shares of her hobby.
Life in the early 20th century was rough. Although Pasteur's work was well established, antibiotics had not become a part of common medicine, and the antibiotics that existed were primitive narrow spectrum drugs compared to the marvels we have today. As a result: The strep infections we commonly deal with today were potentially fatal in 1900. Bronchitis was a rapid pathway to pneumonia, and the latter was often fatal. Tuberculosis was a real killer and not a rare inconvienance. Diseases now unheard of, like typhoid and paratyphoid preyed on adults and children alike.
Common middle aged ailments of today like adult onset diabetes, or high blood pressure were death sentences in 1900. There was no angioplasty to open arteries, there were no stints, there was no open heart surgery. When the ticker ailed, you became weak and died. Even the things we consider minor (yet common in middle aged people) like gall stones, kidney stones, ulcers, were many, many, many times more likely to be fatal in 1900 than today. And to top it all off, there were no treatable forms of cancer in 1900.
You can pretend that it's the infant mortality rate skewing the figures all you want, and it still won't be true. The truth is we routinely survive things today that we didn't survive 100 years ago, and that's reflected in our life expectancy.
I'm not pretnding anything.
Fact is fact, and the fact is that infant mortality has declined precipitously since 1900.
Most of the diseases you describe, besides diabetes, heart disease and cancer, disproportionately killed children.
And the incidence of diabetes and even heart disease was lower then because people didn't have the kind of diet then that we do now.
In 1900 red meat and processed sugars were treats, not routine menu items.
Basically we have learned to extend most lives from 65ish to 80ish now and we have learned to stop childhood diseases from killing kids.
All of which refutes your earlier thesis - that it was easier to stay married for life if the average married person lived to be 48.
That thesis was already irrelevant on its face, because most divorces do not occur among 50 year olds - most divorces occur within the first 5 years of marriage.