Apple offering the same amount of money back for Series 4 as they do Series 3 is a kind of eye-raiser to me. Especially considering that apart having more storage, a compass, and an always-on-display, the Series 4 is essentially the Series 5. In terms of SOC and all.
I’m sure the trade in units are going to a crusher, and not the resale market. In light of that, Apple doesn’t much care about the actual functionality. In effect they are just taking a functional unit off the used market.