It's true that climate is too complex to predict well. Still, I haven't heard many global warming worriers warn about the threat of a new ice age. It's all about the world actually becoming warmer.

Given that, the real problem seems to be the speed. If it took 1000 years to raise 5 degrees, that might not be so bad. If it's 50 years, the necessary adjustments (to both humans and non-humans) might only happen with massive die-off.

But leaving aside the speed, it's not insane to notice that there is vastly more biodiversity in the tropics, than in the arctic. If you were designing a planet for humans to live on, a little warmer is a whole lot better than a little colder.

This doesn't mean that global warming is "good". But you shouldn't dismiss the positive changes out of hand, when evaluating the future pros and cons.