The functionality is that the Watch is merely a dumb terminal offloading most of the things a person does with it to the phone. In that regard, as long as the bluetooth standards and wifi standards remain relevant, there's no reason that can't keep happening on an original Watch for many years down the road, regardless of the additional improvements in hardware to newer models.
Where this is all going to change is when the watch becomes more autonomous, and even completely replaces the iPhone. At some point when the software is rewritten to accommodate that device, the earlier devices will be left behind, though they should continue to work as they always did paired with an iPhone, just like my original iPhone works the same as it always did. Apple just cut it off from iCloud. And in that sense, it's not Apple that really left my original iPhone behind, it's the developers who stopped updating apps, nor do they offer any new apps which are compatible.
Now the reason why the watch is not like the original iPhone, is precisely because it is largely a dumb terminal interface to the phone, and merely mirrors information the phone sends to it. Such a device doesn't need elaborate support to remain relevant, as long as the iPhone continues to use the same radios and protocols for data transmission.
That's why it's most likely a safe investment -- not because Apple won't be improving the hardware every generation, and adding new desirable features. GPS, FaceTime Camera, IPX8 water resistance, among many other desirable features will make the latest watch one to upgrade to. However, as many watch collectors do, the old Watch should still function just as well, and presumably have a different design, or at least finish, to keep around and wear occasionally to mix it up. Apple needs to enable multi-pairing, but that's inevitably coming, or Apple risks leaving a lot of money on the table.