I suspect vaccines would be the kicker. A lot of those childhood diseases people used to die of in droves weren’t really cleanliness-related - diptheria, measles, mumps, smallpox, things like that were only stopped with vaccines. So the average lifespan would drop because so many children would die. Knowledge of the importance of clean water and handwashing and things like that might save us from cholera and typhoid, and we wouldn’t have a resurgence of puerpural fever, but if you look at that list of things you’ve been vaccinated against you have to realize that you got that vaccine because these things kill people, often children, and they used to kill a LOT of them. Are you ready for polio? Because we won’t even have iron lungs to stash those kids in.
You also have to assume that more women would die in childbirth - no emergency C-section, etc. Most premature babies would die. Heart attacks would be infinitely more fatal, as would many kinds of cancer. No pacemakers. No cholersterol-lowering drugs. No blood transfusions.
For that matter, no antibiotics. Ready for the Black Death? Essentialy, you have to define how modern “modern” medicine is. Do we have penicillin, at least? Injuries will be more fatal because of infection and lack of blood transfusions.
I’m no expert, but I’d say that while our knowledge would help us enormously in some areas (childbed fever, cholera, even physical therapy) things that we don’t even really think of as very modern medicine are going to seriously decrease our life expectancy in their absence. I’ve never had a blood transfusion or a hospital visit at all, I was born in a normal vaginal delivery, and I am a healthy adult… however, I don’t know if I may have died of a vaccine disease, my mother was 38 when she had me, and without glasses as a child I probably would have fallen into a river and drowned. Are glasses modern technology?