In this question, there is a misinterpretation of the term "cell lifetime."
The body's structural building blocks are called cells. One of the body's methods for improving performance over time is to extend cell longevity.
Blood and skin cells are examples of "deliberately" disposable cells. This is because the best method for a cell's function is cell turnover. Skin cells are harmed when they are exposed to the abrasive outside environment. A wonderful method for combating the entropy of environmental harm is to slough them off and replace them with fresh ones. Red blood cells are transient "by design" since they lack a nucleus.
The brain's neurons are tuned for a fundamentally different purpose: memory. The brain's objective is to improve throughout time by building up knowledge and memories. These abilities and memories are "stored" as subtle structural adjustments to the way the brain's neuronal connections are organized. In order to maintain that structure as long as possible, the brain must. Since cells are the only thing that exist, the structure of neurons takes the form of their peculiar cell forms, which resemble the complicated tree branches that wind through the brain. If a neuron dies, it also leaves behind its structure and memories. Therefore, neurons should never die in the brain according to ideal design. This has resulted in several safeguards for this structure, including the blood brain barrier that guards against poisoning and the hard shell of the skull. '
Long-term brain health would be an issue if humans could live longer than 100 years. We would need to make investments in preventing this catastrophic mental failure as well as create therapies to keep the brain "fresh" because the brain did not adapt to endure that long (for example, dementia).
