I used to upgrade about every 2 years because that’s how long my contract was and there was usually some killer feature I wanted. Now, new releases only have incremental improvements so I’m not in any hurry to buy a new phone.
I’ll probably keep this phone for another year or 2 as there is nothing wrong with it, even the battery life is still great.
So I’d say iPhone sales are “collapsing” because of a lack of innovation in the market, cost of living pressures, and handset durability, they just last longer.