Why would anyone want to hike the AT without this as their backpack?
That is a great question. The only retort i have is that the Vapor Meridian is just as light and offers an adjustable fitted back, interchangeable belts and straps, and has front access point. Other than that this is a great pack. The real question is why would anyone hike the AT without a Granite Gear pack in general? I mean Trauma's testing alone vilifies the company as a go big or go home pack company!
Quite simply, yes. Check out Brian Roble's yo-yo of the AT in 2008. He's on the fringe of backpacking using a super-ultralight load for the northbound portion (sub 5 lb base weight), but its definately doable.
All of this goes back to Ray Jardine and his original designs in some fashion or another.