People are already dying from global warming and its consequences. For example the deaths of 868 people are attributed to the 2019 European heat wave ( https://en.wikipedia.org/wiki/July_2019_European_heat_wave).

Also, the problem is not that humans in general are incapable to withstand the increased temperatures, but that the extreme changes in climatic patterns and the changes weather patterns will wreak havoc with infrastructure and agriculture. Any species whose lifecycle is tied to the seasons will face decline and possible extinction. With every species removed from the ecosystem, this ecosystem becomes more weak and fragile. Changes to seasonal weather patterns will make living in some areas untenable, causing local and global migration. And we all know how well established societies are dealing with migration (hit; not good at all).