So, the question becomes: "What is inherent in the human body that causes mortality to not be statistically distributed?" That's the real argument.
The people who live to be 120, etc are already stragglers. Also, there are a huge variety of statistical distributions with fatter/thinner tails, so "not statistically distributed" isn't a real thing...
If you download some mortality data, start exploring it, and looking up previous work you will quickly learn all of the above. Try it out: http://www.nber.org/data/vital-statistics-mortality-data-mul...
Cells have a definite amount of time before they undergo mitosis, and telomeres always shorten with each mitosis.
This puts at least one limit on longevity; there are likely to be others. Evolution exerts little-to-no selection pressure for organisms to survive past the caretaker period; in humans, that's grandparents at most.
mmmh not always, there are at least one exception.
"Approximately 7x10^15 mature cells are produced in a human lifetime and these could be produced in 53 cell generations (2^53 = 9x10^15). In 60 cell generations a total of 10^18 cells would be produced, enough for over 1000 years of human life. Thus it is possible that, even in extreme old age, the mature cells of the body are fewer than 60 generations from the zygote." https://www.ncbi.nlm.nih.gov/pubmed/25459141
It may be that rather than limiting lifespan, telomere length is a result of other constraints on lifespan. There may be no point to making them longer.
This lead me to misunderstand the relationship you were claiming between "This is not how the body operates" and "skin and gut cells reproduce far more often than normal".
Bone marrow for example needs to continuously create Blood cells with a short lifespan. (Adult humans have roughly 20–30 × 1012 (20–30 trillion) red blood cells at any given time, comprising approximately 70% of the total human body cell number.) https://en.m.wikipedia.org/wiki/Red_blood_cell
Thus your bone marrow must create new cells at a much faster rate than average, making the average a meaningless number.
Now you might think you could design bone marrow to minimize the number of generations nessisary to produce that blood, but it does not operate with such efficiency. In large part because fewer cell generations does not mean fewer mutations.
>"it does not operate with such efficiency"
Most likely it does not operate at max efficiency. However, since most mutations seem to occur during mitosis, it would make a lot of sense for natural selection to optimize (number of divisions)/(generations from zygote).
If you have a 1,000 cells that can do 30 generations without hitting programmed cell death then a single mutation that kicks that off can form a large mass without tripping your body's alarms which also share that mutation. Even if they end up as a non cancerous mass that large mass is very likely to cause problems.
On the other hand if you have 1 billion cells that can each do 10 generations and they all formed young you still get the same 1 trillion cell potential, but don't risk that single mutation as those 1 Billion cells showed up at a young age. Further, when some of those 1 million cells start growing uncontrollably they only grow to 1,000 cells before hitting programmed cell death which is a lower cancer risk.
Now sure, the body can play around with these numbers to form a crazy number of skin, gut, and blood cells from some relatively small cell pools. But, it's already playing those games while minimizing cancer risk. So, we don't have some pool of 'young' cells to solve problems late in life because it's to dangerous to keep them around.
PS: People often thing of their body's as kind of a tub of undifferentiated mass. But, cells a body structures are optimized for a huge number of problems that only become obvious with deep investigation.
If this is correct, I think you would likely be right. Do you have a reference?