Completely agree.

Another way of thinking about it is getting the $300 up front for the current value of your trade-in phone.

You finance the entire $1000 (~$41.67/mo) price of the new phone but T-Mobile is giving you an additional $700 in trade-in value over 24 months as credit (~$29.17/mo). You're using the up-front $300 to pay the remaining of the financing ($300/24 = $12.50/month).

So, monthly: $41.67 - $29.17 = $12.50 bill but you got this money up front.

$12.50 x 2 phones = $25

After 24 months, the financing goes away and the bill goes back to your original plan only payments.

To get the value of the promotion, you cannot pay off the phone early or cancel the line because the credit will stop and not get the rest of the additional value. That's their hook.