I put hundreds of miles on a pack that I got for $70 at an REI garage sale, before I decided to get a Granite Gear Crown VC60 (large). It's a good bit more space than I need in hot weather, fairly tight in shoulder season, and in real winter a lot of stuff has to ride outside. I decided on it rather than something from ULA (or even more exotic) because I had an REI gift card, and it looked to be the best fit for my hiking style among the stuff that REI sold at the time I got it.
I have no aspiration to be a thru hiker, but I note that a typical thru hike begins and ends in the shoulder season. I'd be hard put to cram the gear I'd want for early spring in Georgia or early autumn in northern New England into a 45-litre pack, even if I'd have no trouble with a pack that size at high summer. That said, I'm not an ultralighter, partly because I think I'd go slightly daft having nothing to do Out There but walk. (Having other things to do means bringing a few extra toys, and needing the pack space to hold them.)