† I would counterpoint that this is pre-2007 thinking, that one needs a button for each new iteration or new feature. The Watch, like iPhone, is software-driven. The same single button on the iPhone in 2007 is the same single button it has now. Could this explain why Apple has built in that secondary button? You know, "the button below the digital crown"? Are they planning for expandability? Time will tell.
I agree that there are enough buttons and other interaction methods for years to come (as others noted on this long thread, it may already be too confusing with Siri, Touch, forced Touch, the digital crown and that extra button given the small physical space...).
As for expandability and upgrades:
How would upgrades work practically? Assuming Apple sells around 10-20 Million watches a year (that's what most analysts predict), the Genius bar would overflow when it comes to upgrade time.
Sending in the watch by mail and waiting 2-3 weeks to get the upgraded one back also sounds very un-Apple. (Not to mention lost Edition watches and personal data privacy would be an insurance nightmare...)
Also, as I noted above, people want unique designs when they buy high-end and luxury items. Rolex, Louis Vuitton and similar brands keep their unit volumes low on purpose because of buyer psychology.
If Apple sells millions of watches, the design would become stale and boring quick for these fashion and luxury-oriented buyers (since only the bracelet design is really different, the watch 3D form is always the same).
I still can't wrap my head around a $5-10k Apple Watch Edition. If Apple enters these price segments, they could also release a tacky golden or titanium iPhone 6s at $5-10k in 2015, where's the difference?