IMO, the 'new atheist's fatal arrogance' is their lack of circumspection. They think they have arrived at the pinnacle of all knowledge, as have others before them. They are too cocky, & do not have the perspective of what they do NOT know, but glory in the many things they do know. They also falsely assume that many of their BELIEFS are proven facts, when they are not even good theories.
I am talking about the 'new atheists' as described by the OP. I have known & know many reasonable, balanced atheists, who realize their knowledge of the universe is limited, & even though they do not believe, they don't push it on others, like the rabid 'new atheists' do. They are more like jehovah's witnesses.. annoyingly proselytizing their creed as though there are no alternatives. I can admire the dedication to their faith, but confidence is a poor substitute for evidence. Belief is not the same as fact.
The opposite of the religious fanatic is not the fanatical atheist but the gentle cynic who cares not whether there is a god or not. ~Eric Hoffer