Actually, doing the NEXT with Down payment is only 12 months. So you can sell your phone every year. Also, you can pay off the phone whenever you want. Not to mention, since you have two dumb phones so you can upgrade whenever you want, you will save money by doing NEXT 12. Im not going to get into all the details, but unless you are grandfathered into unlimited Data that you are actually taking advantage of, NEXT is the better option, especailly because it seems like you are on a line by it self.
BTW, Next 12 with down payment would actually allow you to pay the phone off faster. Plus your bill with just that one phone would be $120 for your plan. If you want actual details, message me and ill explain where the money is saved.
Now, as for NEXT being the only option, I have not heard anything about that at all. I do know we are trying to move our customers off 2-year, but it does not benifit us as reps no matter what you decide to do. But nothing has been said about AT&T not offering 2-year contracts next year.
Sources: AT&T Sales Support Representitive