the lack of antibiotics and the difference in medical knowledge between 1918 compared to 2020? I imagine the level of medical care would be substantially worse given the average lifespan number you mention.
When you read the treatments on this page, I'm not sure that any of them were around in 1918. My guess is that a huge number of people that are surviving today would not have survived in 1918.
https://www.goodrx.com/blog/coronavirus-treatments-on-the-way/