That's a pretty long life, TBF. I appreciate your concerns, though, and do wonder if there was a better middle ground (maybe a micro sterling engine leveraging the heat gradient from my finger to ambient, ha!).
People are buying Fitbit charge6 products today and those probably have an 18 shelf life and cost more.. so maybe it's not totally left field - although the charge6 isn't advertised to fail so soon lol.