This is not a presumption I made. This is believed by many scientists. Telomeres are just part of the story. There are many other reasons[1,2,3]. The third link is not a paper so very readable.
The cell has many ways to combat oncogenesis and when these protections fail, it tries to go into senescence (SIGTERM) or die(SIGKILL).
Nanobots that would replace damaged cells with new non-cancerous ones are not yet here.
I'm a cancer biologist, so I appreciate the relation to oncogenesis. But you said "It is there for (probably) many reasons". This is what I am contending; aging is simply there, because that is what happened. 'Reason' is a judgment we apply, and it's important to appreciate that much of what exists is not driven by anything we might call "reason". I do not believe that the collection of phenomenon we call "aging" is uniformly the product of adaptive consequence. Notably, we die in part because evolution simply doesn't care, or isn't able, to keep us alive indefinitely.
The cell has many ways to combat oncogenesis and when these protections fail, it tries to go into senescence (SIGTERM) or die(SIGKILL).
Nanobots that would replace damaged cells with new non-cancerous ones are not yet here.
[1] http://www.impactaging.com/papers/v2/n7/full/100178.html [2] http://www.nature.com/nrc/journal/v3/n1/full/nrc984.html [3] http://newscenter.lbl.gov/2008/12/02/cellular-senescence/