Many pets did live long healthy lives. The "average" longevity was lower because sometimes the cat became the prey...to coyotes, wolves or other carnivores. Plus medical advances in vaccines helped to all but wipe out distemper and other highly-contagious diseases which would control the cat population in overly-congested areas by wiping out all but those with the strongest, most resilient genes. Outdoor animals face greater risks.The difference between then and now that cats are living longer than they were in the 50's. You didn't see cats live to 17 years old, let alone 20. A lot of times they did die from disease found in the prey they caught.
I dare say that you'll not find much data to support any widespread early death from food back in the days before commercial pet food.