What if the human lifespan were 120 years?

Right now, our lifespan is only about 85 years.
But what if human growth were delayed by 2 years, aging started 5 years later, and the subsequent rate of aging was only about 2/3 of what it is now? What if we lived to be 120?
In that case, geniuses like Zhuge Liang, Einstein, or von Neumann would have lived much longer, leading to far greater advancements for humanity.
We could have postponed concerns about population decline by 40 years.
And perhaps the depopulation of rural areas wouldn't have happened like this.
From the perspective of an individual's life, we could have lived more leisurely, enjoying our youth while trying out many different things.
If we could just be a little more free from the constraints of age, we would feel so much more relieved...
Why is human life so short, and why does youth pass by in an instant? It is truly a pity.