For an iPhone, there likely would be a cut off point for me where I'm not going to spend a ridiculous amount of money for something I upgrade annually. The iPhone is a commodity, given though how we all are completely immersed into it in our daily lives. I don't believe the iPhone X Plus will be anything near $1500. I Believe the next iPhone X plus will range around $1250 for starting price.
Even for $1250 I'd have to consider keeping the 8 Plus another year and upgrade in 2019... assuming the hardware upgrade is minimal and I'd be looking at the same screen size. It's not as if the 8 Plus or the X aren't going to be up to the task. Hardware today is so advanced be it cell phones... tablets... or computers and can easily last for years.
I'm of the opinion that Apple products are already grossly overpriced (although you totally get what you pay for and that is a quality product) but my serious question is... When will the price increases end? Not as long as Apple's loyal fanbase continues to pay top dollar on a yearly basis.
We might not see $1500 next year... but I'd bet my life we'll see it within the next 2-3 years.