The mere change in weather patterns means that assumptions underlying human civilizations for millennia are not true anymore. You will (we already are) see vast migrations with people resisting the migrators leading to more war and strife. Cities built on the assumption of the presence of clean water will no longer have that clean water anymore leading to the destruction of the city, it’s economy and the lives of the people in it.
The rapid change in climate is a huge problem even if theoretically the climate was somehow becoming more suitable for humans.
The earth is currently in an very normal climate fluctuation.
We're almost right at the mean of the past 500 million years.
Humans aren't impacting the environment even close to as much as natural climate fluctuations.
Most of Earth's history also didn't have humans. Or animals, even.
Fortunately over the course of Earth's history, the planet managed to develop a climate that was favourable to human life, and now we've gone and destabilised that.
It's not that Earth is immediately inhospitable to human life, but it is disrupting everything that we thought that we could count on. Countries that developed around an abundance of water suddenly have droughts. Other countries get way more water than they're used to. Or it gets more extreme: droughts followed by massive floods.