I think a more specific way to say it is that weight usually gains you durability up to certain threshold. It is usually safe to assume that a 4lb backpack is going to be tougher then a 2lb backpack of a given volume, but above a certain point (4lbs? 6lbs?) there is a massive drop off and very few 8lb+ packs are any more durable then a lighter pack. I guess my main contention is that this threshold may be much lower then normally assumed. Another example would be Eberlestock, nearly always heavier then any of the high end packs, but no real gain in durability (and I would argue, sometimes a loss in durability). This does reinforce COlineman78's three-axis model as the price on the eberlestock is lower. I'd say the cost/weight/durability (pick 2) thing is more true then not, but never forget that it is entirely possible to strike out and only get one out of three, or even zero out of three. Typically these products don't last long once people figure out how terrible they are, but they can still waste a few hard-earned dollars while they are out there.
I would suggest that a better comparison would be a quality floorless tipi vs. a hileberg, both are expensive and made from quality fabric. They probably have similar durability, but the hillegerg is going to be heavier because it has many more features. These features could be absolutely invaluable on some trips, or an annoyance on others.