One good defense is that hemlocks grow near water so that aids in their ability to defend themselves. It's like the southern pine beetle from 2000-2004 east Tennessee lost ~75% of all yellow pine. Insects are a hard beast to fight.
One thing I would add is that the Hemlock adelgid is a non-native insect. I have read where the Park hopes to save approximately 30% of the hemlocks. I think they have a tough task ahead of them, I have been in lot of areas in the Smokies and driving along Newfound Gap Rd. tells the story. I would guess that less than 30% are still alive and if they are, they are barely hanging on. It takes a few years for the adelgids to kill a mature tree. Hopefully, they can save some in the drainage's, but I think the forest has been changed for a long time, just like after the chestnut blight in the early 20th Century.