I found it interesting but superficial. The SWR doesn't account for a lot of life changes, but then (except for the last couple years, where medical might take a big chunk) a retiree's life is generally a fairly smooth ride.

On the savings side, that happens very differently: people (generally) start off at very low salaries which increase over their 40 year career. They have kids, who take a lot of money just for growing, clothes, college, and so on. So the "savings rate" could be vastly different for a 28 year old than a 48 year old, and is much more highly dependent on "life circumstances" than a simple straight line formula or calculation might suggest. I'm not saying it doesn't pay to "start early", just that employing the marginal utility of money concept, you probably have a lot more to save later than you could ever hope to earlier.