I tried a used iPhone 3GS back in... 2012 just before iOS 6 debuted. I kept the device for a total of about 3 months and then sold it.
IMHO, you are polling mostly long term users here. I hope you are drawing from other sources to form your sample as this forum might skew your results away from the norm and leave you with a non-representative sample.
I keep eyeballing the SE. I think they have reached a more favorable cost/benefit balance with that one. Maybe in a couple months I'll spring for a gently used.