Well I've been saying for a while that if all you want is an ultralite sack, use a poly grocery bag or two and tie them the end of a stick over your shoulder.
If you want to carry anything over 30 pounds, it needs some suspension to put the weight on your hips, and it needs enough rigidity to keep its shape and enough strength to not rip out. Its not nice trying to carry your gear in your hands when a pack rips. Finally at 40 plus pounds, some of the weighty objects may have hard edges that do not conform to a human body and something is needed to pad/transfer this weight to your body. Every piece of gear is a trade off, and very very few reasonably priced packs actually have ul weight and great load carrying capacty. Because of this, one needs a couple of packs, like a summer pack and a winter pack.
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.