So here's my slightly crackpot theory (but, not a far fetched one):
The reason it's "Apple Watch" and not "iWatch" or just "Watch" or something is that it's a product line much like the Apple TV.
It's not a product you're supposed to think about upgrading, if you have the best, or whatever. It's just a connection, an accessory, a tool. Notice that there is nearly zero technological differentiation in the *ENTIRE* Apple Watch product lineup. It's Apple's largest layout ever. Despite being just material/design combinations, it's being set out as a huge list of products and nothing about it significantly differs in tech.
Well, Apple TV is the same way, sort of, after the first gen. Every box identical, basically. The feature set pretty much the same. The upgrades relate more to what is cheap for Apple to produce at the moment than anything else. They don't want you to think about the tech. Just, get the accessory and let it do its job.
Okay, so that is the comparison, these are two similarly named, similarly non-technical products despite being filled with technology.
So then, what's the most practical concerns in both cases? That they continue to integrate with your other stuff. Same connections, same shapes, same remotes, etc.
In the watch world, consistency with band connection types is valued. More than the actual tech inside for practical purposes, anyway. So I think Apple is going to stick with this exactly connector in these two exact sizes. I think, even if they change the design a bit, it will continue to be compatible with existing bands using the same connection.
In fact, I don't expect the design to change, at least, not the shape, for maybe 5-6 years. I realize that seems "crazy" for Apple, but, this is not a tech product. IT's a fashion product, and it's a different set of concerns. People who buy that gold Edition watch may very well continue to wear it even if it's no longer technologically competent, because, it remains a status symbol. But, it would be even better if it continued to operate, and be compatible with new band colors, and so on.
Technologically speaking, the watch design is heavily skewed towards a light weight client design. Sure, some stuff will run natively on the watch, but, the focus in API and functionality design is on preset capabilities configured by heavier software on the phone. And all of the advertised value can be accomplished in this manner. What will likely be upgraded over time are interaction models, but, not performance. If we get a v.2 next year, it will probably come with some S2 chip that uses less power to do the same things. Focusing on the practical improvements, using the same cases, maybe one new metal, and being as "fast" as before.
Anyway, I probably could explain this better if I took more time, but, the idea here is that Apple actually wants you to keep this for a long time, spend money on a few more bands, and maybe upgrade the case once every 2-3 years. The device is an iPhone accessory and will drive commitment to the platform. It doesn't, itself, need to be a high churn product line. Apple can approach that form the style angle and that does not require pushing forward on the mechanical interface. They can even make it thinner someday and keep the band connectors as is.