It turns out I've been shopping poorly for backpacks all these years – but I didn't know it until speaking with someone at MEC. We were shopping for a new backpack for The Boy for his coming school year, because he's starting at a new school that has a pool. He'll be carrying heavier stuff (including wet towels and swimwear) so we needed something larger and purpose-designed.
I learned two interesting things.
First, you can successfully spend less on a backpack if you're willing to recognize that it's a short-term investment. This may seem obvious, but it turns out that there are brands that offer "lifetime" warranties, and there are brands at the other end of the spectrum that purposefully make nothing they expect to last for more than two years. In this case, we went with the latter because daily carrying of wet swimming stuff sounds like a sure-fire way of trashing a backpack regardless of its build quality.
The other news: how crucial the design of the diameter of the backpack, especially the diameter of the opening. A wider opening makes the whole thing less stable and less able to hold things securely, but at the same time allows much better access. When it comes to a school bag, this trade-off works in your favor.
This drove home for me how misplaced some of my backpack choices have been, over the years. I've got one bag that I was planning on retiring, now I'm just going to get rid of it. It had a narrow design, and a narrow opening, and was never really suited to any of the uses for which I'd put it over the years: shopping or fishing.