You have far more faith in elected representatives than I do.
When the dems are back in charge they'll go at the Fair Tax with hammer and tongs. Since no IRS will exist, the quickest way to raise taxes is to convert the Fair Tax into a VAT.
It's not a matter of "faith" at all.
Once the FairTax is law it will have a VERY large constituency in the form of millions and millions of taxpayers who then realize for sure that it was indeed a "good thing" and allowed prices to be lowered and takehome pay increased - not to mention relieving us of the IRS.
A VAT type of law would drastically change all of that and have the effect of raising prices as well as administrative expenses and political mischief which cannot be present under the FairTax but presently is in many VAT countries.
A VAT is, by nature, a greatly hidden tax system while the FairTax is a greatly visible one. After enjoying the FairTax benefits, the politicians would be tarred and feathered to even try to institute a VAT.
With the FairTax, Big Brother has left the building.