It probably actually costs them money, when all is done, to take that machine and get rid of it in a reasonably responsible way.
I use a nine-year-old Air almost daily, but I don't think the vast majority of people would consider it viable, let alone pay for one.
But if you want to talk about much newer machines, Apple doesn't offer good trade-in prices, so it still doesn't look like they are trying to beat out the secondary market.
I think it's pretty clear the trade-in offers are a convenience they can provide more than anything.