That "$X to cover all expenses in lifetime" figure is impossible to calculate, and is not under your control at all. It will be inflated away without your consent, and requires constant evaluation and re-evaluation. My philosophy is to always be investing in capital, whether it be equity, political (relationships with people), or resources.
My spending is not impossible to estimate and for the most part it is very much under my control. If what you said is true then no one could ever reasonably retire at any age.
The relative security afforded by various factors in developed countries (especially USA) since after WW2 up to now may not be available going forward. The biggest challenge is the increasing proportion of non-working people to working people, at least partly due to aging and fewer children.
The part of spending you can't control and which happens to be the largest expense is medical care, which is not a problem right now due to Medicare, but it's already stressing the system and we have quite a ways to go before the aging population maxes out. Retirement ages are already going up and up, and I can definitely foresee rationing of medical care as we push the system to its limits.
The closer you get to retirement, the more obvious it becomes about how much to save to reach the goal. Until then, it's better to just try to eke out as much savings as possible than estimate your spending. Estimating lifelong spending is quite similar to modelling global economic activity over a long period of time.
