Well in reality they will with what ever is in place,sales tax collection would be somewhat more complicated than a flat tax,and has room for cheating,cash sales?? think about the FED's chasing you around for simple cash sales,could get ugly,flat tax on income,while having problems of its own would be less paper work.
So long as the income tax exists, politicians will not resist the urge to tinker with it. How are they going to tinker with a sales tax? Are they going to have business owners determine your income and charge you accordingly? The possibility of tinkering with a sales tax is several orders of magnitude less for a sales tax. All one has to do to know that is look at how sales taxes are currently implemented.
Almost every state and local municipality in the country has a sales tax, and none of them have anything resembling the IRS. the idea that a sales tax would be more complicated than the income simply doesn't pass the laugh test.
Will some sales escape taxation? Sure, but a lot smaller perventage than currently escape paying income taxes. How much business is done in cash? Most legitimate businesses wouldn't even consider not collecting sales taxes.