@timrichards still carrying an iPhone 6s here. I really really want a better camera, but just can't justify it when the phone still works fine. May upgrade to a 2020 SE this year, but can't think of a compelling reason to do it at any particular time.
@timrichards true, there's an element of timing the secondhand market. More on the buy side for me (mine's worth nothing to sell).
@jroper The only thing that can be an incentive is the resale value of the old iPhone if it's in good condition. I've scored $300 or so for my old iPhones in the past. This one's distinctly older though.