Small question: Do people still buy $800 phones, and why? If you care about security, these probably won't receive security upgrades for more than a few years (S3 went from ICS to Kitkat. S4 went from Jelly-Bean to Lollipop, S5 went from KitKat to Marshmallow, S6 went from Lollipop to Nougat, so I don't expect this to make it past Android Q[1] (which should come out in two and a half years)), and unlike older models, custom ROM support is quite bad for modern Samsung models (S6 Lineageos just came out. S8 isn't supported yet).
Wouldn't it make more sense to get a slightly worse quality phone, but upgrade it every two years or so?
[1]. Unless Treble will make it so much easier to port to S9.