That's why I imagine Apple will somehow market a trade-in program. As Gruber writes, the gold watches will retain a lot of their value just from the metals in the case and bracelet.

Yea but not $5k worth. The Watch according to my calculations might contain about $1k in gold. So if they sell it at $6k like Gruber thinks, they won't be able to give you back much of that on trade in. Most people would probably just keep the watch with those prices.