Damage in the genome causes cancer, which is the only type of genomic damage that we need to worry about. Cells that undergo other types of genomic damage undergo apoptosis - programmed cell death. All of your other cells still contain intact copies of your DNA, so you don't need to worry about DNA damage in single cells unless it relates to cancer. Do you see what I'm getting at?
Metabolism is extremely complex. If you influence just a single variable, there are many different effects. I agree. We should not tamper with metabolism because we understand so little about it.
However, the metabolic byproducts, i.e. damage that accumulates within and outside of cells, are fairly well categorized and understood. If we can get rid of the damage, and thereby let metabolism do its job, we don't have to intervene in any process. We just have to clean up the damage.
The reason antique cars are still running is not because people have changed how they work. Instead, they simply replace and clean up the car periodically to keep it in working order. We'll do the same with our body on the cellular level.
Well maybe not the damage in genome, but a certain "shift" (or lack of it, or just lack of epigenic instructions/blueprints, from a certain phase. Instructions, that would enable us to sustain health/youth.) maybe what it's all about. As the experiments with fruit flies show - it's not the change or mutations of new genes, that enables them to live a lot longer, but the change of expression patterns of the same ones, is what's important. So basically, if the certain kind of "pattern" that grants us health in an early adulthood starts to "blur" (because of lack evolutionary pressure) and we start going downhill - the manifestation of that "blurring" is a wide variety of what we call "damage" (not like damage is the CAUSE, but more likely - a RESULT. At least I start to get get such an impression.). And to "fix" all of these "damages" (and they go up in number faster and faster) may be quite an unlikely scenario, as like you say - if metabolism is extremly complex - so probably is the "damage" that happens to it.
The same study showed that a significant part of "change" happens in expression of genes, that are responsible for energy metabolism, so some kind of a mitochondrial intervention may be one of the succesfull apprach "Fix-damage" style (if there's a mechanism - a lot of stuff may go wrong, but there may be a common variable, like Oil, that lubes it, and if it dires out - the whole mechanism starts to overheat, etc.. And mitochondria COULD be such an "oil", an universal variable in a whole organism, that could add up to the overall effectiveness of the function of parts that depend on it..). Just a guess.
Anyway - we can witness that just one, single intervention can make it or break it. So maybe it's not the actual mechanism of aging that's so complex, but it's manifestation (you know - like with that equation of a fractal, that's very simple from a first look, but it can produce an amazingly complex looking structures.).
Well it's just my rambling, in part to just better arrange the thoughts in my own head.