Can growth continue?
So I disagree with his argument that infinite growth is possible. But even taking that premiss as correct 
If you want more brain power you don't need more people, you can simply treat the people that already exist better. Think of how much brain power is lost to poverty and inequality. People who dont have enough money to survive, people who are in debt, spend so much mental energy just trying to survive. Calculating how much they can afford, how they can stretch their money. Its such a waste, when we could just provide them with the necessities they need to live a dignified life. And then we could see what they created. Sure some people make it out, but percentage wise they are the exception that proves the rule. 
Based on what we know about epidemiology, the impact of pollution, the impact of prenatal and early childhood stress. Just consider how those compound with years of poorly funded education in neighborhoods made unstable by the transience of poverty. Add in the school to prison pipeline (partially fueled by a rise in police officers in schools in response to school shootings) and (imo) inevitable substance abuse and think about how many brain cells have been destroyed. I dont think I'd be able to think deeply in those situations. 
If brains/ideas are our most important resource we are wasting them. On an almost unimaginable scale.