That's the deal breaker for me, and why I would never support FT. I'm not buying the "Amend the Constitution again to prevent reinstatement of the income tax" argument. Just because a thing is explicitly prohibited by the Constitution doesn't mean Congress won't pass it, the President won't sign it, and the Supremes won't let it stand.
For reference, see "Campaign Finance Reform."
It shouldn't be a "deal breaker" at all since if Congress were going to lay both types of taxes on us at once they would have certainly already done so in the almost 100 years we've suffered under the income tax.
Truth is, the American voter wouldn't stand for both at once. regardless of that, though, the FairTax bill actually ELIMINATES not only the income tax (along with several others) and the pertinent parts of the taax code, but it defunds the IRS asd sets a date certain for any remaining work to be done and defunds the IRS to boot. It also requires the destruction of income tax records and calls for the elimination of the 16th amendment.
Wit all of that, there is no way any politician will be able to snap his fingers and put another income tax in place (Charley Rangel has tried recently and failed and we don't even have the FairTax in place yet - which gives us some protection from that).
The best assurance of never having an income tax in our lifetimes is for the american people to hee the benefits - financially and otherwise of having a tax system like the FairTax).