There might be a few people who believe that. However, a little coaxing and question asking will bring most people around to understanding that we're merely severely fucked.
Even this minor step then admits to the possibility of some mitigation, and that's a good thing.
Climate change is not going to wipe out humanity or life on earth. It could, however, completely reset or destroy what we currently consider "civilization", and for most people (especially those who particularly enjoy the comforts of said "civilization") there may not be not be much of a distinction between that an extinction. In addition, while losing only 1-2B people due to climate change is a far cry from human extinction, it is still a very big number. Even 400M would still be a very big number, with very substantial consequences for migration, resource control etc. You think people living in the tropic-to-tropic latitudes are going to just sit back while the outer latitudes live it up? I don't think so.
As for the timeline, that's always hard to say, but I'm almost 60 and I find it likely I'll see some severe disruption in the tropic-to-tropic zone before I'm done with this mortal coil.
Global industrial civilization is driving the sixth mass extinction on earth. Every species that goes extinct is gone, forever. If you’re a member of one of those species (or depend on them for your survival), it’s an existential problem.
Already there’s been a 75% decline in insects, over the past 30 years.
When all the pollinators die out, how much longer will humans survive?