Marginal effect on INDIVIDUAL fitness, but a huge effect on THE FITNESS OF ECOSYSTEMS. Without aging, ecosystems would be unstable dynamic systems, and would not last more than a single generation. Without aging, everyone would die at once, in famines and natural disasters. Aging is necessary to spread out the death rate, so that populations don't swing wildly up and down.
This way of thinking assumes that all life is evolved for maximum reproductive fitness. This is false. It is bedrock evolutionary biology, but it is also false. It turns out that the way evolution works is much more subtle than the postulates of R. A. Fisher's system, which dominated evolutionary thinking during the 20th century. Here is a good read on the subject: http://aeon.co/magazine/science/why-its-time-to-lay-the-self...
The tradeoff between reproduction and aging is sometimes present and sometimes absent. The successes are over-reported and the failures under-reported, so you might get the impression of "extremely well documented" with a cursory reading. Here is an example from Cynthia Kenyon's lab, "Healthy Animals with Extreme Longevity" http://www.sciencemag.org/content/302/5645/611.full
The most important thing to keep in mind is that your own body is your best experimental lab, and that you should pay attention to what works FOR YOU and keep a diary. Other people's experience is good for giving you ideas of things to try, but people vary considerably, and you will find your own experience changes from week to week, as well as evolving over time.
The Selfish Gene was a phase in our understanding of evolution. The mainstream now agrees that evolution is a whole lot more complicated than that. It's still true that "group selection" is a dirty word to a majority of evolutionary scientists, but this is shifting pretty fast - much less true than it was 10 years ago. Take a look at the works of David Sloan Wilson. I recommend his book, Unto Others. http://www.hup.harvard.edu/catalog.php?isbn=9780674930476
There is no "natural selection" within an individual body, within an individual lifetime. Yes, there are some cells that continue to live and others that continue to die. But the cells are not competing, with the fittest winning out. Rather the whole scheme is orchestrated centrally for the good of the genome (which is the same in every cell).
Tragedy of the Commons is exactly right.
Yes, the genes spread through the gene pool, but then the population goes extinct very quickly because there's a huge population and nothing to eat. This is the most powerful form of group selection, and it was first described by Michael Gilpin, http://press.princeton.edu/titles/663.html
(I'm not endorsing this view - I believe aging is programmed, largely based on the fact that genes for aging have been conserved since the dawn of eukaryotic life, i.e., cells that have a cell nucleus.)
Yes, it is group selection, and group selection has been badly maligned for almost 50 years now. But a large body of literature confirms group selection. See David Wilson's book, Unto Others. Group selection is especially effective in the case of population dynamics. See Michael Gilpin's monograph, Group Selection in Predator-Prey Communities.