And perhaps a link to the particular post you mention escapes you, too?
The 20% devaluation mentioned is not going to occur and there have been many examples to show this (including some on this thread). Overall the disposable personable income for most taxpayers (including those with legacy savings from the former tax system) will be greater, not less.
This is partly due to the prebate which lowers the effective tax rate considerably even for someone spending at twice the poverty level or even more and the fact that you may invest this money and receive untaxed gains thereby boosting the amount. It is only if spent for taxable items that the money is taxed. The income it earns can accumulate untaxed.
Since prices will drop with the advent of the FairTax anyone also receiving wages will have a large wage increase in addition. Even after adding in the FairTax after the price decrease there will be little if any purchasing power decrease. It could very well increase and there are ongoing studies to try to assess this.
Claiming that no one has addressed the point, however, is not accurate.