It's not a lease, it's an installment plan. There's nothing stopping you from paying off the remaining balance on the device and selling it yourself. The early upgrade program just makes things easier if you don't want to deal with selling on ebay, CL or trading in to Gazelle, Amazon, etc. Basically you're trading higher resale value for convenience with the upgrade programs.
You're not saving $500 every two years because you're buying outright. You're saving $500 because you're not replacing your phone every year.
Either way that's still $500 regardless of whether its a lease or not. If you'd like I could re-do the math assuming I sell the newest phone after 1 year to afford the next one.
I'll get back ~$600 per phone and selling every year. that's $1200 bucks, one phone is free since the price of the phone is ~$900 after 9 years of phones I'll get both phones for no added principle investment, if you'd call it that.
However, the other way around I'll only be spending $432 on a phone each year on a leasing plan.
So Lets say I sell it for $600, subtract that from the original $900, and that's $300 per year on a phone. After 9 years that remainder of 300 catches up and I get the second phone for no additional cost.
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Your math isn't entirely correct though.
IUP: iPhone 6s 64GB + AppleCare+ = $878
$36.58 X 12 = $438.96. That means, Apple is giving you $439.04 for your current phone on trade-in when you upgrade to the next phone. By paying off the remainder of the loan, you are essentially selling it back to them. I will gladly take $439 for my iPhone 6s without having to worry about fees, scams or the hassle of selling it. Plus I get to keep all the accessories for free. Which is valued at $75 if you were to buy them from Apple.
The savings you are talking about is only if you upgrade every other years. Not yearly. Also, you won't be able to sell a 2 year old iPhone for $500. iPhone 6+'s are worth like $400-$450 tops right now and the 7 hasn't been announced/launched. That price will drop at least $100 in a week's time.
Check out the pricegeek.com - http://www.thepricegeek.com/results/iphone+6s+plus+64+gb+unlocked?country=us
Since the phone is mine and not tied to the service contract, I can have it unlocked at any time, $400 and $500 is the difference between locked and unlocked phones. I also don't use AppleCare with my phones.
My math works, and in my previous comment I describe how it works even on a year to year basis.