There is a large circular opening in the back of the watch for a reason. Sensors will be added to that area. Miniaturization will only allow more technology to be fit into the same size case. Tooling is also becoming more and more flexible. There is no reason they can not support several different platforms of watches into the future.
Having a gold watch priced at $5000 is more about fashion than being pretentious. In a world where there is $30k - $100+k watches, a $5k watch isn't really that pretentious. Maybe it's Lexus level pretentiousness, but it's not a status symbol like a Rolls Royce. Women especially want to coordinate. They aren't going to want to wear a aluminum or ss watch with their existing gold rings, necklaces, bracelets and earrings. Even middle class women often have more invested or inherited in those other items. A matching smart watch that will be upgradeable for a long time at that price will be something they consider. The edition will likely be sold at jewelry outlets and be discounted just like all other high end watches.
For them to sell, and for apple not to look elitist, they will inform us about their upgrade policy upon their release event. I expect these will be upgradeable for decades. They only released as much as they needed to generate interest which it has. They wanted to show us the design rather than leaked from their Asian manufacturers. We rarely get the whole story this far ahead for a new product.
Just because apple hasn't offered upgrade paths before, doesn't mean they will always continue in that direction. They have called this a very different personal device, and have given far more attention to the case than the guts. There is one gut, at 2 different sizes, each in 3 different materials, each with 2 different colors with dozens of different bands. So one computer, two different screens, and hundreds of design combinations. In order for success, they knew this had to be a high quality, well-designed, long lasting watch first, with the functionality of evolving upgradeable componentry.
Gold and stainless steel watches tell time no better than a plastic one. What do you see more people wearing? Form over function describes the watch industry. Apple is promising both.
It is really quite obvious when you consider what they have done. Everyone agrees that a $5k gold watch that would have a lifespan of 2 years would only be acceptable to the 1%ers. Apple has always been interested in moving a lot of product to the masses. Isn't it more likely for them to make a shift to an upgradeable device, rather than shift so much of their attention to an elitist device? Apple is not as stupid as so many of you may think.