The 6S Plus was $750 for the most basic model at launch and now ebays for about $200 which squares with losing about half the value per year which is pretty normal.

Consider also the fact that the battery life will be substantially decreased from new and that most people wont be able to replace it themselves.

Something that is overprice is actually more likely to decline because 2 years from now its just a comparatively slow device that no longer serves as a status symbol.