Of course the discussion is about thrift. The watch isn’t (shouldn’t be) a downgrade year over year. If you don’t care about the cost and whether the new model provides value proportionate to that cost, you will get something slightly better than what you had… it’s an iterative update after all. There’s not much interesting to discuss there.
That said: here’s two reasons why I’d suggest you shouldn’t buy every year, besides thrift.
1: it generates ewaste. It doesn’t matter if you re-sell the old watch or Apple recycles some of it. Waste is generated when the product is produced, it just might take some time to get there. Your purchase is a signal to Apple to make 1 more device destined to eventually become waste.
2: Capitalism. You are speaking with your dollars. If you mindlessly upgrade every year, you’re telling Apple that no matter what they do, you’ll buy it. That doesn’t incentivize them to invest more in development to create a better product.
I kept a series 2 until this year because they weren’t delivering the feature I want (battery life). I bought a refurbished series 6 this year because of wanting a sensor to monitor a health issue. I’m still not buying a new watch from Apple until they deliver on battery life.