Well, being completely honest, with iPhones in it's current price point I'll only upgrade my current X if Apple introduces a really big bunch of innovations like cameras with better night mode, better zoom and more customisable setups; a new design (I like the current one, but to spent more than thousand bucks in a gadget identical to my actual one is a big NO option); a much, but much improved battery (having used a 6T for some weeks now I just realized how poor is the iPhones battery); a better mobile network speed connection (I was impressed when I used my 6T and my X in the same Vodafone 4G network in Berlin... The surfing speed was almost 2x faster on 6T than on iPhone, every single day, for the entire month I was there; back to Brazil the experience turned even worse, since the 6T surfing speed in Vivo 4G in São Paulo was almost 3x faster; I obtained similar results comparing with my wife's XS).
So, a lot of things, really. Don't get me wrong, I really like iPhones and use them since 2007, every single new model. But since the X I really don't think I'll be even tempted to upgrade it sooner than later. If we would talk about new releases in Android world, for another side, the conversation will change completely since 2018 marked, in my opinion, the first really innovative year in this market in a long time, which I believe will continue in 2019. In a much, much lower price point in vast majority of cases.