I've never replaced a smartphone out of search for novelty, lack of updates or repair difficulties. Rather each of them got progressively slower over time eventually reaching a point of being unusable. And I'm pretty sure I have high tolerance for slow UI - whenever I asked my wife to do something on my last one, she would think it's not working at all.
How much of that is intentional on the vendors' part is an open question, but I'm sure they aren't losing sleep over it.
It's even worse on iPads. They nag you incessantly to update to the latest OS, but woe betide you if you do. The new OS eats too much memory so there's nothing left for other software, and countless background processes are killing the last bit of performance. But they still rate this device as “compatible” with that OS. Apple sure knows how to squeeze the last pennies out of unaware users.
I was having a similar issue with a ca2015 Macbook Pro, new things wouldn't work without updating the OS to whatever the new flavor is. It now happily runs EndeavourOS...
This might have been a problem 10 years ago but the situation is pretty good now. I've got an ipad air 2 2014 on the latest OS available and it still works really well. My Apple Watch is 3 years old now and still feels as fast and good as day one.
iPad Pro’s are ridiculously overpowered now and I’d recommend them for a long term solution. I’ve been using a 2018 iPad Pro with the A12X Bionic (from the iPhone XS and XR release) and it’s been running without a hitch on latest iPadOS @ 120hz. I imagine an M1 iPad Pro will still be more than enough to run whatever iPadOS releases 5+ years later.
It seems like hardware has greatly outpaced software requirements these days. I imagine phones with 2023 hardware will be more than viable in 2029, whereas a phone purchased in 2010 would definitely be a piece of junk compared to a phone in 2016.
How much of that is intentional on the vendors' part is an open question, but I'm sure they aren't losing sleep over it.