Quality of life is often measured in economic terms: How much money people have or spend, and the monetary value of their assets. Research indicates that people’s satisfaction with their lives is not very well correlated with wealth (at least above a level of basic subsistence).
If we measure quality of life in terms of happiness (personal satisfaction), then it makes sense to maximize those things that influence it most. As my personal research indicates, happiness is strongly correlated with freedom and life expectancy, while more loosely correlated with consumption (ecological footprint).
Combining longevity, population, and quality of life, we would want the product (multiplication) of population and happiness to either increase over time or level off at some high value.