Manufactures do not change the design or materials used in backpacks often. So unlike apparel where you’ll find items in end of season clearance sales, packs are carried over for a few years. REI has a couple sales a year where you’ll find good discounts on the brands you mentioned. The brands listed still make good packs. A number of years ago the “go-lite” movement in the industry changed up the materials used in packs a bit. Packs might be slightly less durable but for most users this is not an issue.