I can see the confusion. I'll use real numbers so people can see what happened.
Last month, my bill for three lines was $228.xx. At the beginning of this month, I switched to the new family plan - $100/mo for data, $40 per line, but because my plan was more than 18 months into it, we basically get a $25 per line credit, so it's $15 per line (as advertised). On top of that, I get a 24% discount, so my bill dropped about $60 (insurance for each phone is some of that $40 difference that isn't apparent, as my bill is $110.xx + insurance x3 and telenav (which I can probably get rid of), but also fees/taxes).
If I were to get a new phone (Unless I purchase it out of pocket and activate it), I would lose my $25 a month credit (my line would be $40 instead of $15 base). AT&T Next, however, does not remove that $25 credit. So for the cost of adding a new phone anyway (without the upfront cost), I'm paying $29 a month more than I was after the price dropped, which is a net increase of $4 if I were to get a new phone with a two year contract, except I paid zero down.
Yes, if I want to trade up at 12 months, I have to trade in my current phone, but I'm not paying activation or the cost of the new phone, and if I like my phone and keep it for 20 months, then it's mine to keep.