I've just read on Reddit that At&t is planing to increase 5 dollars per line on old UDPs again in March 2017. It was confirmed there by two At&t workers:
https://www.reddit.com/r/ATT/comments/5ltp9e/legacy_unlimited_data_plan_price_increase/
https://www.reddit.com/r/ATT/comments/5ltp9e/legacy_unlimited_data_plan_price_increase/