You'd be saying the precise same thing if our natural span was 30 years, or 200.
Well, yeah.
That is, you're using our current state, to explain why our current state exists.
Woah there, that's a big misread of what I was saying.
I'm not using the "state" (the number X of years that is the currently observed natural lifespan) to "explain" the value X. But rather the intrinsic design and function of the system as a whole.
There is nothing "correct", "intrinsic", or "functional" about aging. That's conflating evolution with lack thereof. We evolved to sustain a reproductive cycle and propagate ourselves. That evolutionary optimization has no direct bearing on the specifics of how we age. What we know about aging so far indicates that it's an accidental byproduct of many side effects of the functions of our systems, including the hormonal system that governs puberty, the immune system, the metabolic system, and the higher order DNA management systems (epigenetic programming and replication). None of those side effects are in any known way selected for by evolution, and most of them seem to be tunable to achieve far more durability or regenerative capacity out of our bodies than we are currently used to. Multicellular aging is a collection of random side effects that vary between species - there are many animal species that are effectively immortal.
My point is, you are inside the system, using values from the system, to explain the system.
If humans lived 200 years, but went infertile at 50, you'd be explaining how the system was designed to entice grandparents to care for grandkid's babies, because they were infertile and couldn't have their own, and because it would improve their genetic bloodline's chances.
I still feel much of your reasoning was using what is, to explain why is, because what is.
This response applies both the above and its sibling comment, which are quite valid in asking that I should perhaps clarify further. So if I may:
Yes, our current "operating mode" (with an apparently built-in expiration date at roughly 2x peak fertility) is but one of (infinitely many adaptively stable configurations ("local minima" as it were, in the fitness landscape). It is by no means the only possible (let alone the best possible). As the above comment points out, we can easily imagine other operating modes that are also viable and stable.
But the basic point I was trying to make is: though it isn't the only possible mode of operation for our species -- it is also far from random, and most likely the result of many interlocking tradeoffs in our basic genetic program (which we are just beginning to map out and understand).
The upshot being -- my hunch is that, while we will probably figure out how to tweak some of the dials, attaining a substantially different mode of operation ("longevity for everyone", basically) will require changes substantially beyond tweaking of dials -- or a "patch" here and there. Most likely it will require something akin to a top-down rewrite (or that is, a comprehensive rewrite of many interlocking mechanisms at once). Which will require not only conceptual advances, but to be blunt, "trial and error" (live trials, most of which probably won't work out so well for the test subjects).
That is to say -- qualitatively different from merely deriving a "patch" to our system sufficient to, say, obviate the risk to certain diseases like dementia or cancer.
Well, yeah.
That is, you're using our current state, to explain why our current state exists.
Woah there, that's a big misread of what I was saying.
I'm not using the "state" (the number X of years that is the currently observed natural lifespan) to "explain" the value X. But rather the intrinsic design and function of the system as a whole.
There's a huge difference there.