The automatic adjustments are made by government statistics which are calculated by unelected bureacrats. The rate adjusts without any action by Congress. What you don't understand how much reported wages will go up after the fairtax is enacted. For instance, a typical self-employed person reports only a small salary and has most of his profits flow to him from an S-Corp. This lowers the SS taxes he owes. Under the fairtax there is no motivation to report lower wages, so people will increase their wages to maximize SS benefits when they retire. Under the current system you have to pay 15.3% to do that, under the fairtax it is a freebie. Reported wages will go way up, and thus so will those SS/MC rates in the fairtax and thus so will the overall fairtax rate.
Go look up the percentage makeup of the SS wages that will go "way up" and show how this offsets all increased economic activity.