If a Presidential Candidate Dies or Is Incapacitated Before Election Day
electionlawblog.org
electionlawblog.org
[1] https://upload.wikimedia.org/wikipedia/commons/4/47/Excerpt_...
[2] https://www.semanticscholar.org/paper/United-States-life-tab...
Though I suppose there is some selection bias too: if you can run for presidency at 74/77 etc. you must be fitter than the average person your age; be it because of better health, better genes, or easier life hitherto, or something else still. So your odds of making it are probably better too.
Most people that age I know wouldn't be able to keep up with the electoral calendar pace.
Further, this assessment also assumes the 74/77 year olds who died as a consequence of infection were all of average health, rather than randomly sampled across the health spectrum (which us a fairer assumption). I don't think there's any basis to conclude that they were average health.
We do know Trump has some complicating conditions such as mild obesity and unconfirmed heart disease: https://www.google.com/amp/s/amp.cnn.com/cnn/2020/10/02/heal... If anything, these elevate his risk.
Even the US civil war had long reaching effects on other countries - and were far more connected now.