Because people can now create private nest eggs for retirement and/or big medical bills without that nest egg account being subject to income taxation (that's the feature of the Steve Forbes flat tax proposal and FairTax), it tremendously reduces the financial pressure on both Social Security and Medicare/Medicaid. And that means we can make major reforms of entitlement programs to reduce its cost.
So the entitlements can remain in place once income tax reform is instituted?
Both are the problem.