I repeatedly looked at the AT&T Next plan. I compared the total monthly cost of my service on the subsidized plan vs. the total cost of service if I had AT&T Next. Without including the price of the phone through Next, the Next plan was exactly $10 less per month. That would equate to savings of $240 over a two year period. However, the cost of the iPhone was $27.50 to $42.50 per month in addition to the plan. That meant that I would be paying $17.50 to $32.50 per month more than I currently paid depending on the phone I chose. It was not a $15 or $25 discount. And I did not calculate $450 in additional charges over a two year period. The truth was that I would pay more money using the Next model.
AT&T hasn't always offered the $15 / $25 "Mobile Share Value Discount". That said, AT&T Next, as it was first introduced, *WAS* a worse offer even with the discount. But, AT&T had to improve Next because like you, it was figured out, known, and no one was shutting up about it (most notably John Legere). But, I also forgot to include a class of post-paid customers in my list. Out-of-contract, now month-to-month, or in other words contract completed customers who also get the $15-$25 discount, as well. (except probably the grandfathered unlimited data ones. AT&T will do anything to try and make them switch)
Don't believe me checkout the † text when you purchase: "Smartphones on AT&T Next, month to month, purchased at full price or BYOD also receive this discount. If you upgrade to a new 2-yr agreement this discount will be lost."
Completing your 2-year contract moves you to a month-to-month customer and gains you the discount and "upgrading" (contracting for a new iPhone) removes the discount. So we're not even comparing Contract to Next anymore. We're comparing Contract to completed Contract. As an on-contract customer you are the only class of customer to pay the full "Mobile Share Value Device Access Charge".
$450 (your subsidy) / 24 months = $18.75 / mo
So why isn't the service discount $18.75? That would just be to totally transparent, then wouldn't it? But, also because then they couldn't use the service discount as weapon / incentive. If you buy 10GB data per month we'll give you $25 off. If you don't we'll penalize you a bit.
AT&T Next and the "Mobile Share Value Discount" was only introduced as a response to T-Mobile killing their subsidy and contracts and overage fees, paying ETFs, Data-stash, Wi-fi calling, etc.... So that said, your understanding does have some merit. Before that, they never gave any discount.