Why remove hunter achievements and the associated bags/other rewards associated with them because you added more mobs after the fact? Why not grandfather us in if we already completed them?
Wait, they remove the tangible rewards? That seems silly. I can see removing the achievement but not the physical rewards.
You still have the backpacks. You'll even get the same one again (move to shared bank before claiming). I have 3 brineslayer packs, and since they messed with it, will get a 4th =P