There's an enormous difference between total extinction, and civilization being wiped out; that's my whole point. You're trying to conflate the two, and they simply are not the same. For most of human history, there has been zero civilization, and certainly no "critical infrastructure". Humans can go back to that. It won't be pretty, but it is possible, and it's not like extinction.
A temperature increase exists where earth will become uninhabitable by humans. One study mentioned above estimates that temperature increase is 12C. It could be higher or lower, but we don't really know without further studies on the topic.
Before reaching that temperature, life on earth will become a lot harsher for all due to the effects of climate change, and we should be doing all we can to avoid those harsher conditions, regardless of what we think the upper bound temperature for humanity to continue existing is.
