Flat tax will never happen. the RICH will never let it happen. let's face it, they control everything and if they did indeed want it, they would get it.
If the bottom 1/4 of society (let's say currently paying NO taxes) suddenly has to pay 15% in taxes... and it's safe to assume they don't save much and spend nearly 100% of their income..
Then 1/4 of society would suddenly be spending 15% less. Walmart, shoe stores, restaurants, etc suddenly see a 15% drop in sales, automatically. Companies barely scraping by suddenly see sales drop by 15%. Good luck with that. Will never happen.