Update: Apple Support informed me (via two separate chat people, for some reason), that a new device isn’t eligible… because the company that they use to do trade-in doesn’t list it until its successor comes along.
I don’t know if this/these people had good/accurate info, but that’s what I was told by Apple representatives.
Ludicrous.
Between this stupidity, and the still-missing Touch ID on flagship iPhones, I guess I’ll stick with this SE3 for much longer than I anticipated.