“The Fair Tax would replace all federal income taxes...”
Unless the XVI Amendment is repealed this is never going to happen. If the XVI Amendment is left in place some future Congress is going to pass legislation re-establishing an income tax.
Every fairtax proponent I know of endorses getting rid of the 16th amendment. In fact, the fairtax bill itself endorses the idea. But if you think that they are going to repeal it without the replacement tax system in place first - well I've heard of a bridge in Brooklyn that someone is trying to sell.
Once the Fairtax is in place, repeal of the 16th will be a relatively simple matter as millions upon millions would demand it!