If I understand correctly you're saying that people take better care of their phones because they want to be able to sell their old ones when they upgrade? Whereas presumably people who were just going to keep the old phone wouldn't look after it as well and would then break/damage it sooner?
I'm sure some fraction of people probably do what you day, but as far as I know, it is known that this is the exact opposite of what happens for a lot of other people (even iPhone users): they in fact tend to be more careless and break them more easily because they want to upgrade. [1]
Anecdotally, on a somewhat related tangent, I have found it curious that whenever I see a phone with a broken screen, it almost always tends to be an iPhone. I have no idea why this is, and I doubt it necessarily has to do with the above effect, but it does make me skeptical of the idea that people take better care of their iPhones because they presumably want to sell it later.
I'm also skeptical that most people go through the trouble of selling their old phones at all, even iPhones. Far, far more people seem to have iPhones than bother to try to sell their old ones, so I would guess a lot of people just do nothing with their old iPhone. Or trade it in. It's not exactly painless for everyone to sell their old iPhones.
Moreover, incentive-wise, like every phone company, Apple has every incentive to make everyone upgrade. Despite the continued support for old devices, you can imagine they pour a lot of time and money into making sure this happens, and judging from how well they've done, I'm not under the impression they've had massive failures on this front.
So I mean, it doesn't seem impossible that what you're saying could happen, and I'm sure it does for some subset of the population, but every evidence I've seen suggests it's far smaller an effect than would be necessary to compensate.
[1] https://www.smithsonianmag.com/smart-news/when-people-want-u...