I'm not really surprised, smartphones kinda hit this point of "good enough for most people's purposes" 3-4y ago and short of an actual reason to upgrade like the 4g-5g switchover there isn't a lot of incentive for most people to throw down $400-1k for a new phone every couple of years.
I would have happily kept my OnePlus 7T for a few more years if the network switchover didn't require new hardware.
Personally I don't need a faster smartphone at this point, if anything motivates me to buy a new one it's usually better radios, better battery runtime and better cameras. The rest of the gewgaws don't matter much for daily use.
It seemed the last incentive they were trying to get people to upgrade was throwing as many cameras as possible on the back. Companies need to try and innovate again and the folding screens is at least one way of them trying something new. Prices have also climbed dramatically so I'm not surprised people are bothering to upgrade.
Also I would add inflation went up, prices jumped. Meaning not so much free spending cash any more. People might have previously had the cash to update phone, just for sake of update even without it being necessary. Now days? People have way more important things they have to spend more money on.
This market stagnation was what got me to buy a Fold. Every 3-5 years of the same size slabs, just imperceptibly "faster". Then came something new finally. Same as my pixel 2xl, I'll have this till the battery or screen starts to go.
I hit fleaBay and bought a used 9 Pro. All I really wanted out of the upgrade were newer radios (5g + AX wifi) and better cameras. I think I paid around $350, if my track record holds I'll keep it for 2-3 years then do the same again for the same reasons. I've been halfway looking at a 10 Pro/T or an 11 model for better battery runtime (Snapdragon 888 is a bit of a battery hog for the performance) but I don't really have a reason to upgrade yet.
It's more tied to the change of the business model.
Phones used to be subsided by the plan and the 2 year contract lock in, so if you didn't upgrade every cycle you were effectively leaving money on the table.
This is why the market accelerated so quickly compared to any other class of hardware.
As the 2 year contract fell out of favor (thanks largely to T-Mobile), you had 2 year heavily discounted payment plans tied to device trade in that took their place, but these were opt-in as opposed to the previous model which was built in to every contract.
While the economy was strong, the depreciation on your current device and effectively FOMO on maximizing its trade in value kept the system driven at similar numbers.
But as purses have tightened, suddenly the outlay on increasingly expensive devices with lower trade in values for past devices is a racket people are opting out of.
It was never really about features as opposed to status and reup indicators. Most of the rest of the world has more like 3 year phone replacement cycles for the past decade, and have been fine with the same feature parity per model year as US phones.
I'm honestly surprised the 2 year thing kept going as long as it did.