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I can't imagine there will be too many people upgrading from a 2017 X to a 2018 X, there just isn't enough improvement to justify the hit you would take on reselling the phone and the price of the new phone. I think the majority of people with an X that are upgrading are doing so because they want the bigger screen (myself included). If there was no X+ coming, there's no chance I would have upgraded this year.

Oh yes there will be. The iPhone X will get people to upgrade.
 
If you current phone is working fine - why upgrade? Keep it till it drops and then buy. I am hoping to keep my 3 year old 6S+ for 3 more years; just in time for the 5G roll out. Phones aren't getting any cheaper to be much of an impulse buy every year (for me)...
 
here in Australia the telcos are just starting to talk about a switch over, but it's in small areas first. I'd say we won't get it here until 2020
 
While 5G is being rolled out slow and to selected areas, it might take 2 years till it’s virtually everywhere.
 
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