Generally, if the frameless pack is constructed in a way that distributes weight evenly across its seams and stress points, there's no reason for material failure after X number of miles. Packs wear out from the following reasons:
1. A seam was stretched beyond it's breaking point because you carried too much. This doesn't change over time, it's a constant, except when we start talking about #2.
2. UV damage/fiber damage. Some fibers and materials like silnylon will begin to break down from exposure to sun and water. Some do this slowly, like, on the scale of decades, and others break down a bit quicker. There's also such a thing as bending strength; fibers like wool can be bent tens of thousands of times before breaking, but synthetic fibers usually only get a few thousand. On points on your pack that bend with every step you take, you might see wear after 3,000 miles or so. Maybe not even then.
3. You did it. Dropping, scraping, throwing. General abrasion from use. Rips because you pulled too hard on the shoulder strap in the wrong direction. Etc.
So, to answer your question in a roundabout way, McHale packs last a very long time because weight is well-distributed, the seams that undergo the most stress are reinforced, he chooses fabrics that have low decomposition rates, and his packs are generally free of flaws like weak spots. An UL pack can be designed to distribute the weight of the pack evenly across an integrated foam backpanel, or across a shoulder or waist strap, and most are. Cheaper ones don't distribute weight across enough stitches to ensure that the load is underneath the breaking threshold for the stitch, and you get damage over time.
What does this mean for your ULA pack?
It's well constructed to redistribute weight, so if you're a few pounds under the max load, it'll keep performing indefinitely. You will likely drop it on rocks and sand enough times to wear it out by the time you complete several thousand miles, which I suspect is why most people replace their pack.
Hope this is helpful.