People produce amazing things, like art and science and technology. If we had 10x the number of people (roughly 100B), we could produce amazing things at 10x the rate. And all of those amazing things would be shared across the entire population, so it’s actually a 100x improvement: 10x the number of people enjoying 10x the amount of cool stuff. That’s 10x the production of Einsteins and Feynmans and Ramanujans.
Of course, people think 100B people would be some kind of dystopia, and it would be if we handled resources like we do today. But that’s only because we’re complete fucking dumbshits about absolutely everything. The reality is that we can give 100B people an American quality of life if we were just better about how we did stuff. Mainly, that means much more energy use–mainly from solar. Energy solves every other problem, whether water supply or food or transportation or recycling.
As one example of how stupid we are, if all we did was convert the land used for corn ethanol (a totally counterproductive attempt at making fuel more sustainable) farmland to solar panels, the US would have ten times the amount of energy production. Not just electricity. Everything. That’s more than enough on its own to make absolutely everything (transportation, etc.) electric and still leave plenty of room for growth.
And to counter @Zoobi 's unfounded accusation, I was posting about this 10 years ago. Or even longer, albeit indirectly.