Realize that you're cherry-picking the most expensive configuration for the SS, with the link bracelet. That band is $500 by itself, and anyone who has one could continue to use it with their new watch. They're not going to spend $1200 every time they upgrade. The SS starts at $599 for the 38mm, $649 for the 42mm.
Look, I get where you're coming from. I spent over $700 for mine since I got AppleCare with it, which I felt was a good use of money for a 1st generation product. I promised myself, and my wife, that I would wait at least 4 years before upgrading. The point I'm getting from reading this thread is that most series 0 owners who are upgrading are doing so for a specific new feature, either water resistance or LTE. When we bought the series 0 we all knew these would be likely additions in the near future. I honestly don't care about either feature, so I'm not upgrading.
Your situation is different. You're considering the SS with all its current features and wondering if TODAY's watch will last you. To answer that you have to ask what features are likely to be added from here. I think the most likely candidates are an always on display, a built-in camera, better battery life, new fitness sensors/features, and thinner and/or new case design. In the time frame you're talking about I think most of those will be added at some point (if they don't add a camera it will be because Apple makes a conscious decision not to, not because they can't do it). So what you need to be asking is, are any of those things that will make you wish you had waited? If not, then I think it would be safe to say that the SS Apple Watch could last you 5 years or even longer.
Aside from new features, the only other limiting thing about the Apple Watch is diminished battery life over time. But realize that all watches require some kind of service at some point (either a new battery, or a tune up for mechanicals). Let's put things in perspective. I have a nice Tag Heuer watch that my mother gave me as a graduation gift. Every time it needs a new battery I have to take it to a jewelry store to have it done because all the seals in it need to be pressure tested once the battery is replaced. Most stores charge about $80-$100 for this. Want to guess what Apple changes for a new battery? $80.