I refuse to pay $200 for AC. I think that's crazy. I don't want to own my X in year 2 and 3 with no warranty.You’d rather make smaller monthly payments on an 8 than finish up buying out your X ? Why?
Having the phone be “yours” wrapping up X seems more economical long run than restarting payments even if lower.
Instead I will most likely purchase the 8 next fall at full price this fall (let's assume $600) and get AC, which I'm assuming will still cost $129. At that point I'll use the iPhone 8 for 3 years. Yes I'll end up paying $229 more versus keeping the X but I'll also have a second year warranty and a newer phone in year 3.