> But if you are not a big believer in retirement, then ...
Unfortunately, "believer" or not, modern Western medicine has gotten extremely good at keeping infirm elderly people alive.
From a Capitalist medicine PoV, that's optimal for extracting their wealth.
From a macroeconomic PoV - "retired" or not, the net economic input from the top decades of the demographic curve will be nothing so positive as you seem to assert.
Not at a whole-population scale level to any real significance though. There are just an abnormally large number of baby boomers, exacerbated by the subsequent massive fall in birth rates following them.
It's a "law of large numbers" problem - if you have a big population, you'll just have more people who live longer within that population then a similar, but smaller population.
US life expectancy has actually been falling somewhat recently - the whole "people are living longer" thing has always been a massive over simplification of long term historical effects and pressures (i.e. life expectancy didn't change all that much with the discovery of medicine, but it changed a lot because infant mortality stopped dragging down the average).