Firs, let me say that I know of not a single FairTax supporter who does not also support repeal of the 16th amendment but, as a practical matter, there is simply NO way it can happen as you suggest. Congress is NOT going to move to repeal the 16th amendment without first having a viable replacement tax in place.
Secondly, if what you say were true, why do we not have them both today as there is currently NOTHING preventing it?
Just posting a warning, I’d like to see meaningful tax reform as much as anyone. I just don’t trust Congress.