you don't have to upgrade, but it used to be tempting
It's just a portable computer, and people don't buy new computers every year or two.
It's not just the phone though, you would think by now they would release a smaller watch with round face.
In the iPod era we got a new form factor almost every year. You didn't have to buy it, but you had the choice. Now if I want a round watch I have to buy from google, which doesn't work with my iPhone?