If your 6+ is doing fine for your needs (not what others project on the internet for their needs) - then I wouldn't recommend buying any new phone until your 6+ fails to do so. If it just needs a consumable replaced, such as a battery, that is still cheaper than buying an entirely brand new phone. A new phone costs the full ride regardless of whether you have the 6+ today or in a year or two, there is not "upgrade" pricing on hardware. So it would be wise to squeeze as much utility out of the phone you have for as long as possible, than buy a new one.