Personally the turnaround time on new device updates has caused me to
stop believing that I can expect to use the same device three years from now.
I used the original Pixel for 3 and a half years and at this point it's gotten completely unbearable to use. If it wasn't so I wouldn't have felt forced to upgrade to a phone lacking the features I wanted. Maps takes half a minute to load, the share dialogue is O(n) since the list of targets is regenerated every time you open it, the screen runs at 30FPS or something, taking pictures has several seconds of delay between shutter presses and a quarter of them come out as a solid green, and there's just general slowness of several seconds on nearly every app. And that was after I downgraded to LineageOS from Android 10, when I was getting four hours of standby time (!) and a hole burnt in my pocket.
The thing was as smooth as the OnePlus 7 Pro I'm using now at the time I got it. It was criticized at the time for being overly expensive and an iPhone clone. And now, using it in 2020 feels like using a $20 knockoff device. It was released in 2016. Three years isn't terribly long in the span of human life - at this rate I'll have to buy about 17 new phones if I live another 50 years just to be able to use them at a bearable level of performance. (Certainly a lot of potential money to be extracted for the manufacturers.)
Three years later I'll have to remember this post to see if this still holds true for this new phone.
Also there are practical reasons for this besides social ones (the insistence that OS and hardware refreshes are pushed out yearly or so at the cost of increased CPU and battery usage for existing devices). Flash memory has a limited lifespan and I would imagine it's incredibly cumbersome to replace. It's one of those things where you would just buy a new phone with the money it would cost.
Reusability in the phone market is severely lacking. I run Linux on a ThinkPad manufactured a decade ago and have zero issues running 2020 software. You can't realistically expect to do the same on a smartphone that old. PostmarketOS can't become polished soon enough - I just need to be able to make calls and can't do so using it yet.