Let's start by agreeing that there is some upper limit to the number of people the planet can sustain. How about a trillion people? Could we agree that that would be too many?
Then the next question is, who really wants to live on a planet that has maxxed out it's capability to house humans? Do we really want to have cities 10 times or 100 times bigger than - say - Mexico city? Who wants to live in a place like that? Do we want to squeeze out all other species, and eat algae gruel all the time?
Not me! If some virus came along and made 90% of humans sterile for a generation, and so we got back down to under a billion people, I think the planet and our environment, and all the critters we share the planet with, AND the 700 million people left, would be far, far happier.