So Garmin, Polar, Suunto, Timex, all who make $400+ models and typically replace those models every other year and which avg. users replace every 4 years... they are nuts too? All of those watches are 100% plastic.
The Apple Watch is a hybrid sport + activity + daily use watch. It starts at $350, not $500. $500 is for the stainless model, which includes a stainless band, not a plastic one. It's the equivalent of getting the $500 metallic paint option on a luxury brand car.
I don't think most people who buy an Apple Watch are going to upgrade annually any more than they upgrade their MBA or iPad annually. Truth is most people don't even upgrade their phone annually. Tech moves fast, but not that fast. Very little difference in year over year models. It takes at least two years for it to accumulate to be noticeable.
I would agree with your views, but I would ask you to remember one thing regards what you said.
Products start out, develop fast as things are learned, then over time reach a certain level of maturity and the development curve starts to flatten off.
Cars, iMacs, iPhone, Macbooks, even iPads etc, are well well along this curve, and whilst still improving the curve is a lot flatter than at the start.
The Watch is, or will be at ground zero.
It's not a mature product that has had the bugs and kinks of years and millions of users ironed out of it.
If the watch was on version, 4, 5 or 6 then perhaps I'd say yes, the current v5 watch it probably good for some time, the early issues were corrected, the battery life is better, the screens been improved, the sensors are more accurate etc etc...
But we are at the very start of version 1.
So I'm not sure you can yet, at this point draw any comparisons from how long many keep other Apple products that have gone thru many years of improvements.