Pretty much every current T-Mobile plan rate is only promised through 2014 or 2015 as well. Most plans go up $10/month after the promo date ends. Same with that free 200MB/month in data - that has an end date too.
Incorrect. T-Mobile's rate isn't a promotion. What WILL change after January 2, 2016 is the high speed data allotment (the line above which you drop down to EDGE speeds) drops back to 1 GB per line.