The combination of solid gold and the obsolescence period of wearable electronics is uncharted territory.
Luxury mechanical watches need service too, it's a very standard thing in the industry. Apple can offer a CPU/screen/sensor improvement every two years for about a thousand dollars for the luxury models and owners would pay it as they pay the water seal replacement for their Omegas and Rolexes... only Apple's margins are going to be sky high because it may be an easy thing done by a grunt in a tech service shop, and luxury brands have to send your watch to highly skilled engineers sometimes in another country.
EDIT: I posted before finishing the article, Gruber thinks Apple may upgrade the electronics too, but for "hundreds" of dollars.