$1000 for iPhone X plus $200 for AppleCare. $1200 spread over 24 months is $600 per year. You trade in your phone after 12 months and the value of that trade is $500 (half the cost of the remaining payments, not including AppleCare).
$1200 for phone and AppleCare up front, which means it’s still $600 per year. But iPhones used for a year can’t be sold for $500. If you want to upgrade every year, you’re losing money.
But let’s say you decide to keep your phone for two years. Ok, so how much to 2 year old used iPhone go for? About 1/5th new, or about $200 for your old iPhone X that you bought outright. Would you pay $200 for a new iPhone Xs or whatever after a year? If yes, you should use IUP. If no, go ahead and buy your phone outright and keep it as long as you can stand.
Buying your phone outright doesn’t make you good with money, it just means you plan to keep it long enough that it will save you a few hundred in the long term. Or, I guess you could just be bad with math I suppose...