Of course, I'm not an expert on this subject, and I know that the process of aging is not well understood even today.
As I've also said in another post, I do not intend to make the process of aging and death a simplistic one with only a single factor. Multiple things come into play and eventually the effects add-on to bring the organism to be unable to maintain it's activity any longer.
I did not intend this as a model of aging, but only a model of one of the factors of aging.
As you and others have mentioned, there are mechanism in the body who prevent as much as possible the accumulation of mutations as we grow older (controlled death, etc.). These would be analog to some sort of 'natural selection' inside the body.
The 'inbuild limitation of cellular replication' is aso interesting, although I have not much knowledge on it. But I would wonder why such a mechanism would even develop in evolution. In oher words, what is the evolutionnary advantage of it if its main purpose is to limit our cellular replication and make us die ?
Also, I would also think that stem cells are not exempt of mutation accumulations as we grow older, which would lead to the breakdown you mentioned, I have no idea of how stem cells replicate and if they do in an asexual manner (well, probably I would guess), so maybe they can avoid mutation accumulation.
Finally, this is not my field of expertise in french, let alone in english, So my knowledge as well as my vocabulary is limited and I would like you to consider this when replying on this subject.