Why does it need to be bigger than 70 liters?
My Six Moon Designs Starlite was fine at Philmont. By the specs, it is a 67l pack, but a bunch of that is in the outside mesh pockets. The main bag is 50-55 liters, depending on how far you extend the collar.
Most of our Phimont had 65 liter or smaller packs.
Also, "strong and durable" sounds like "heavy and takes abuse". Modern materials are very strong if you take reasonable care, like setting the pack down instead of dropping it. My pack weighs just over two pounds, and Philmont was its third year on the trail and its second long trek.