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.