Who isn't? If the phone doesn't break, it becomes unusably slow with any operating system that still receives security updates (if there are any, which is not a given). Also, I have my doubts on whether it's actually possible to reliably delete all data from your old phone as a consumer.
I never trade in. Every old phone I've had has seen more use than the value of trading it. Most go to a relative then back to me for some secondary use. Plus it's good to have a backup device in case something happens to your main phone.
It's the same as trading in a car versus selling it on the private market. It is ALWAYS a bad deal. It's also not like if you trade it in that's more environmentally friendly, because they just recycle it immediately, and most of the carbon emissions come from manufacture. They're just trying to inflate prices by killing the second-hand market.
If you're that worried about your data, screen who you're selling it too. But unless you're a journalist, politician or very high up in a business, you're probably fine.
But is it better than someone else getting use out of the phone for several more years? Because that's what I'm suggesting, and it seems you're arguing against a strawman...
What strawman? I'm talking about my old phones in my drawer, and I'm certainly not the only one who does that.
The thing with that is that nobody can get (much) use out of my old phones, because I already use them until they're broken or unreasonably old - when even aftermarket ROMs like LineageOS don't get security updates anymore, it's time to retire that phone, and I regularly use my phones beyond that point.
I don't. Maybe I should but I like having an emergency device on hand. That said, I'm due an upgrade in a couple months, so if a trade-in is meaningful, I'll trade my eldest Pixel.