Desertification, wet bulb temperatures, heat stress, loss of food in the environment, ecosystem collapse due to climate change happening too fast for ecosystems to migrate, and the massively increased range required to find food causing them to not find mates.
And capitalism demands the “business as usual” path that is also the worst-outcome-possible path.
Wholesale civilizational collapse has already been modelled to occur between 2035 and 2045 to an extremely high degree of confidence, with a 50-80% collapse in human population to occur before the end of the century. You can’t maintain a post-iron-age civilization through that degree of collapse.
I would even posit that a post-bronze-age civilization would be difficult enough to maintain. Because you need a fair amount of tech to work iron on scales larger than a blacksmith’s shop.
And these models do not include major attributes of climate change, such as wet bulb temperatures forcing billions to migrate into first-world countries to survive, or how regional ecosystem collapses cause trophic cascades that ripple around the globe. They only looked at simple things like desertification, topsoil loss, chaotic weather (unreliable rainfall) and so forth.