Mountaineering pack materials

> I'm guessing it's due to 500D's tighter weave and lighter weight?
Yep.
Note that some Montbell packs have silnylon as part of the body. That's a long way south of 500 d Cordura.
It all depends … on how much scrub and how much rock you are going to encounter.

Cheers

I have found the origional 400d "Pack Cloth" to be more than up to the task for multi decade use, even in Joshua Tree and High Sierra granite. The 1000 denier stuff is good on the bottom and
lower 6".

If you like buying new packs every few years, use something lighter.

Mike, the 100D (blue) material on the Thruway isn't traditional ripstop. It's a "nailhead" weave with a textured outer surface. Quite tough for the weight. I tested one for all of 2012 and found it quite satisfactory. It won't hold up to lots of hauling or anything, but should be sufficient otherwise. The DX40 on the bottom is tough, top shelf stuff.