We need both.

Sustainability is defined by the amount of resources that a population can take from the environment without permanently destroying it. For a bigger population that amount of resources that can be used before reaching that threshold is smaller by person.

Just imagine a tribe living of a fruit tree that gives 10 apples a year. Maybe a tribe of 10 individuals can live of that tree but a tribe. But what happen when the tribe grows and suddenly there’s 100 individuals trying to live of a 10 apple tree? It’s illogical to take population out of the equation, because it’s one of the biggest factors, the second biggest factor is quality of life (how many apples we eat a year), and the only factor that you are considering relevant is the one with the smallest impact that is how efficiently we recollect our apples. That last factor is the one with the smallest impact in the whole equation, and it’s the only you seem to consider to solve our problem. We, of course, need to be efficient because it cost nothing, but efficiency by itself is not solving the whole problem.

Your own equation and your own logic is supporting my argument that we NEED to reduce population.

The only thing left against it is the dogma.