Annual upgrades are a luxury no doubt. Very rarely are their YOY improvements that warrant it, but if you make enough then who are internet strangers to judge your decisions?
I'm on the upgrade program currently because I realized that I upgrade from an iPhone SE to iPhone 8 to iPhone X? and if I was going to upgrade annually on my own then it was cost advantageous to go through the plan. My justification, for now, has been the camera improvements. The quality of my iPhone camera essentially dictates the quality of my memories down the line as I don't use an external camera. The better that camera is, even marginally, the better quality my memories will be down the road.
I already look back to the pre-2011 days before I had my iPhone 4s and think about all of things I wish would have captured on camera, but had no convenient means to do so.