> total loss of thymic reticuloepithelial tissue and the associated thymocytes should occur only at the age of 120 years in humans.
The thymus involution is the most obvious hard limit for Ageing beyond 120 years old. At age 70 your thymus has 100% shutdown and the only T cell immune system you have left are the old one still remaining in your bloodstream) (the half life is of 17 years)

The involution of the thymus (3% per year) has no consensual explanative theory behind it but the two main theories are:
1) a fact is that thymus involution appear by biologically controlled mechanisms, it appear too soon (teenage years) to be mainly due to a normal effect of an accumulation of damage (aging) although reducing oxidative stress reduce the rate of loss of thymus.
Another fact is that it is universal among mammals, and since a long time it is a conserved evolutionary mechanism.

Now I'd like you to appreciate for a minute how lucky we are and how absurdly kind life is with us. We are optimized for survival in a world with scarce resources.
We should have developed atrocious mechanisms that would make the human condition deep shit for increased survival but we have mysteriously not. E.g after taking your meal of the day we should go into low energy consumption mode, have much reduced muscular and cognitive abilities) except in the presence of dangers/stressor, until the next day.
As I said, mysteriously we are not too much behavioiraly optimized but that does not preclude that some mechanisms (especially ancient mechanisms) have been designed for hardcore resource contraints and therefore have surprising tradeoffs.
The thymus is an energy consuming organ but it's not needed for medium term survival. Because the t cell it has produced will still work for the next 20-30 years.
The age of reproduction in humans is well below 40 year old and evolutonarily speaking, a human leaving past its age of reproduction is an absurd inefficiency. It does not contribute much to the survival of the group and it consumes critical resources. It would often be logical to induce an instantaneous death to humans past a certain age (evolutonarily speaking). Many living beings, especially insects, die after reproduction, usually being eaten by the mother/childs.
Again nature has been kind with us, but basically thymus involution is like killing you after the age of reproduction, except it's a very slow process that reach completude at 70 years old and absolute immune absence at 120.
(note that this is only about T cells, the spleen (producing b cells) is another topic although it doesn't age well either).
So yeah this theory it the most potent to explain why the thymus involute.
Since this theory is a bit harsh for politics, researchers do not talk much about it. They fallback on a milder version where they simply say that the thymus in itself consume energy and reducing it allow for oleer people to consume less energy (or allocate it to other things) without addressing the obvious evolutonnary advantage of controlling lifespan.
Their milder theory is probably true in conjunction with mine. A backing evidence of them is that testosterone reduce the thymus, possibly for resources competition.
https://pubmed.ncbi.nlm.nih.gov/9427047/

"Why does the thymus involutes?"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3295077/#:~:tex...

I am one of the only transhumanist on earth to know that the thymic hormone thymalin restore thymus function and increase lymphocite T production by 680% and that this effect has been tested to lead to a 410% reduction of all cause mortality in humans.
https://pubmed.ncbi.nlm.nih.gov/33237528/
It's non use (e.g it was the trivial day 1 covid cure) is causing millions of deaths daily and induce misery in the quality of life of so many humans. Unfortunately science illiteracy is too strong for mankind to evolve past its infancy.
The same could be said about Skq1.