Engineer-minded people can discover lots and lots of such "obvious" issues in healthcare and of course the answer isn't that nobody would've guessed it, it just doesn't matter. Healthcare (outside of very specific diagnosable illnesses) is 90%+ medicine theater. People expect some pills, they want a few words with someone in a white coat, and their problem goes away usually by itself. Chronic persistent issues without obvious cause befuddle docs and they often don't find anything even after lots of tests.
Our better health and longevity today is more due to better work conditions, better food, better sanitation and food preservation, basic vaccines, basic disinfectants, basics like penicillin, smoking less etc. And the low hanging fruit to improve further is lifestyle: eating/drinking fewer calories (especially less sugary stuff), exercising/sleeping more and having better social connections. None of it is particularly arcane or in need of precise measurement.