What KK's analysis elides is the entire concept of mental and physical decline. We are working with population distributions here, but generally speaking, there is a non-trivial likelihood that your 60s are going to be less productive per unit hour, than your 50s. And your 70s, vs. 60s. KK speculates what if he gets to 99! Amazing what he could accomplish. Yeah well this 59 yo can still do decent endurance feats. But I find it very hard to stay bent over a floor tiling mosaics, a very deep love of mine. And the management has to manage the bending over the plants in the garden.
Of course though, I am still as mentally sharp as I was at age 28, when the string of cool things started to happen. Or maybe not. I have an arsenal of well maintained skills to keep me productive, but not seeing much new to add to it.