While books don't expire like milk and eggs, tastes change. Some titles become classics and old copies gain a "vintage" value. Others fall completely by the wayside.
It's similar with curriculum. When a publisher releases a new edition, one of two things will happen:
Since we buy and sell used books, this is part of what we deal with nearly every day. We try to support old edition curricula as long as we can get consumable workbooks, but will typically discount and stop purchasing teacher materials, unless we can create bundles of materials.
We've begun to make a distinction between this section and our clearance section. If a book is in a clearance section, we're not purchasing it anymore. If it's here, marked (old), we're still trying to support it; if marked (really old), we're working on clearancing the books.