Market saturation, trade imbalances, extremely high prices, gigantic phones with underwhelming differentiation–whatever the cause, I’m leaning on the side of both market saturation and people putting off upgrades because the iPhone is now both extremely overpriced and (in the case of the XR) an ungainly lump of plastic that is not compelling enough of an upgrade at that price.
I have a faint hope this will lead to their cutting prices and going back to smaller SE-like form factors, but considering they are still making ludicrous amounts of money, systematically hiking up prices across the board and seem to be largely deaf to customer feedback, it’s a toss-up as to whether that will happen.