There is a lot of "probably" and "perhaps" and not much actual information. There is no credible information about them "... relatively smooth sailing up into their 70's and 80's."
Here is an excerpt from National Center for Case Study Teaching in Science (NCCSTS) article:
https://sciencecases.lib.buffalo.edu/collection/detail.html?case_id=752&id=752
Seems possible that this is just beyond your intellectual grasp, but from wikipedia:
"Longevity, maximum lifespan, and life expectancy are not synonyms."
Just a quick scan and saw this article near the top of my search. Won't vouch for it, but it makes sense with other things I've read over the last number of decades.
This is the problem with your "quick scans" through unverified and non-reputable sources. You will continue to confirm your bias in the following decades as well by hearing what you want to hear from unknown authors. Your opinions are unlikely to change, you will not consider information that doesn't confirm your biases.