You've identifed the fly in the ointment. Success is predicated on doubling the economy in 15 years. I expect the opposite. 75 million baby boomers with high paying jobs will retire. Instead of putting (tax deferred) income in the stock market, they will remove it (as required by law) to cover living expenses. Most will find their retirement resources inadequately funded. They will spend only what is necessary to get by.
It's almost laughable to think that taxing retirees to extract money to pay them social security benefits is a workable approach. It is cow sucking its own udder to survive. Money to cover such obligations must be acquired by a wealth transfer (taxation) from employed persons to retired recipients. The ratio of employed persons with disposable income to retirees expecting benefits is going to drop radically in about 10 years. Socialism and ponzi schemes never work as intended.
"Success is predicated on doubling the economy in 15 years. I expect the opposite."
You are entitled to your opinion, but none of the economists who have studied the FairTax expect "the opposite". No reputable economist that I am aware of, for example, thinks it would be economically detrimental to eliminate the bias that our current tax system provides to foreign producers.
That's why having a base of consumption rather than income as the tax base used for S/Ss would be preferable and less punitive on income earners.