For me, I definitely can't afford to find ~£500 just to upgrade my iPhone every year. All Apple products are a "considered purchase" for me.
If I consider that my Apple Watch, my iPad mini, and my iPhone won't last forever and I factor in that it's a struggle to replace just one of those in a year, then I've got to be thinking about each of these needing to last a good 3-4 years.
Last year I upgraded my iPad mini, this year it's my iPhone, next year it will probably be my Apple Watch. It won't be the turn of my iPhone again until at least 2023 - preferably not until 2024 or later.