The intent of the Calculator is to offer the comparative information so some bloke spending a given amount under either system can get a handle on his FairTax effective rate. Presumably he knows his effective income tax rate (though I doubt it since hidden taxes aren't considered under the income tax).
But the savings themselves lose no value and even the income earned could also be plowed back into more savings. Your point is not made.And when those savings are eventually spent, they are taxed. Their real value will always be reduced by the amount of the FairTax whether they are spent immediately or saved for twenty years.