Having had an Apple Watch since the Series 0, I think Apple wants people to have a ~3-year upgrade cycle. They essentially update the internals one year, the next they come out with some interesting software features, then the next year they do some external design tweaks. Then they repeat.

There is no reason to upgrade every year, or even every two. It is around the 3-year mark where there tends to be enough new features or your battery life is getting bad enough that there is reason to check out the new watches. Even then my mom uses a Series 6 and is very happy with it and actually uses it a fair bit.