Earlier quoted context omitted.
Considering where climate change is heading, it's surprising anyone doesn't think this.
Climate change is very real and worth addressing, but it's extremely unlikely to wipe out human civilization. It's just going to be costly and kill millions, not billions.
Climate change will make parts of the world inhabitable that are currently populated by humans. It will also result in crop failures, which will cause famines. In some cases formerly native crops will no longer be supported by the changing local climate or farmland may become completely unusable. Potable water will become harder to source. Water contamination is a source of many deadly diseases.
Even if we assume most of the deaths will be concentrated in places like Africa or the Indian subcontinent, the global economy relies on these places for resources and cheap labor. People in ongoing climate catastrophes also don't tend to stay put and die in an orderly fashion, they become refugees or riot against their governments. Things can get politically messy even in the nations next door as humans tend to be uncomfortable with political chaos and mass deaths.
But unlike the Spanish flu, or the potato famine, or COVID, climate change is not a temporary blip that happens and then goes away. If all the carriers of the plague have died, nobody dies from the plague unless they get infected handling the dead. Famines can starve millions to death but once there's another harvest the survivors have food again. Climate change isn't like that. If climate change creates a drought, that's not just a drought, that's now dry season and it will be dry season every year from now.
There are currently 7.8 billion humans. There's absolutely no reason to believe climate change can't kill billions, especially once it's managed to kill the first hundreds of millions.
Keep in mind that it's not about "addressing" climate change. It's not a moldy bathroom tile that you need to clean up or replace before the mold spreads everywhere. It's a fire our entire way of life is fueling every single day. We know what needs to be done to slow it down to survivable levels (or at least levels that are lethal for less than 1% of us) but we can't just pass legislation or appeal to personal responsibility to do that because it involves changes that would be economical suicide for anyone doing it alone. The world economy is playing a game of chicken with each other and nobody is bluffing.
[1]: https://grist.org/climate/how-many-people-has-climate-change...