Usually I replace a smartphone because it has some hardware fault that makes it unusable and it is hard to repair or the cost of repair vs capability is not very favorable.
Even if I was replacing smartphone before they died, I would probably sell them anyway. Do people really have various phones in working order waiting in a drawer?
I will upgrade when my device is many generations out of date. So no one really wants to buy one and I will then buy not the latest but a generation or two “out of date” so it’s cheaper. So I have a heap of phones over the years that no one wants. Not even me.
Usually I replace my phone when the battery is almost dead and a full charge doesn't last more than a few hours. At that point the phone is usable but not good enough to sell. I usually keep it as an emergency backup phone
I never sell anything, everything finds a new life.
I got a Sony Xperia Compact from maybe 2016. I use it as a bicycle navigator / recorder. Recently it stopped reading SIMs, probably rusty contacts, but all I need is wifi to update OSMAnd and send me the tracks I recorded.
I also have a very old tablet, maybe 2014. My SO uses it to view movies with VLC and TV via TVHeadend.
I take my previous phone kayaking. Once I have downloaded the appropriate maps over wifi, I can use it for navigation, logging and photography without being worried by the prospect of dropping it in the water.