Human lifespan

Because of zero average population growth, increasing the life span of people on the planet would have to increase the generational time and thus necessarily reduce the number of individuals, but increase the number of the individual's life-years.

The marginal utility of these extra years may reduce in time, thus longer life span would probably lead to less total happiness.

What is more, with increasing or even infinite life-span, it would be more difficult to maintain reproductive capacity to replace the individuals that eventually do pass away (e.g., in accidents). With a necessarily low fertility, a very long life span would seem a rather instable system.