A watch is a mechanical timepiece you wear on your wrist. The Apple product mentioned is a small computer you wear on your wrist.
Expensive watches are mainly expensive because of the internals, not because of the case. Sure, gold/silver/etc will drive up the price - but a good mechanical watch in a stanless steel case can still cost $10,000 - because of the intricate, hand-assembled internals. Replace those internals with $10 worth of silicon circuitry and a display, and it won't be worth $10K any more, even though there's the same name on the face.
Assuming that all things you wear on your wrist are interchangeable is like assuming all automobiles are interchangeable. Taking an Aston Martin and replacing the engine/transmission/driveline with one from a Ford Focus isn't going to create a desirable vehicle, and it won't be worth $140,000.