There is a problem with being concerned only with longevity: We could theoretically have only one person remaining in the “species” with an extremely long lifetime. To deal with this clearly bizarre consequence, we must also maximize the number of people. Maximizing both longevity and people, we would expect population to either grow or stay constant over time, but never decrease.
In a resource-constrained world (or “universe”), there would be a maximum population, the world’s “carrying capacity,” where the rate of resource consumption and the rate of resource replenishment were equal. The consumption rate depends on both the size of the population and the resource dependency of the quality of life, while the replenishment rate depends on processes in the environment that convert matter and energy into usable forms. To maximize population, we must therefore decrease the consumption rate, increase the replenishment rate, or both. The Universe, as a finite entity with unbendable laws, ultimately limits the availability of resources, so these are our only options in the long term.