The obvious counterpoint to any "hordes of climate refugees will destroy the first world countries" is that borders exist, and machine guns were invented over a century ago.
The reality of modern warfare doesn't favor large forces, poorly organized and underequipped, that are attacking reasonably well prepared defense positions.
Now, is there a will to use all the tools of modern warfare against climate refugees? Currently, no. But if "hordes of climate refugees will destroy the first world countries" stopped being a distant theoretical concern, and became a practical one? If there were real examples of border checkpoints in first world countries being breached by force, with border security overran, and thousands of somewhat armed and somewhat violent climate refugees pouring in through the breach? I expect that to change very quickly.
Well, I wasn't try to imply that the refugees would win, and yes, that's entirely the point because the scenario qualifies as a breakdown of human civilization that Gates doesn't acknowledge. Even if some people will still be able to have their Sunday brunch while their country's borders are bathed in refugee blood.
I think that for most of Europe the 1939-1944 period was mostly living in the dark ages, but yes, you could still get tickets for the Rockettes at the Radio City if you were a middle class New Yorker.
The reality of modern warfare doesn't favor large forces, poorly organized and underequipped, that are attacking reasonably well prepared defense positions.
Now, is there a will to use all the tools of modern warfare against climate refugees? Currently, no. But if "hordes of climate refugees will destroy the first world countries" stopped being a distant theoretical concern, and became a practical one? If there were real examples of border checkpoints in first world countries being breached by force, with border security overran, and thousands of somewhat armed and somewhat violent climate refugees pouring in through the breach? I expect that to change very quickly.