Should the USA switch to a regressive, flat tax?

The USA became the richest, most powerful nation on Earth with a strong progressive tax system. During and after WWII, our top tax rate ran as high as 92%. John Kennedy dropped it to 70%. During the post-war period, we paid down the massive debt of WWII and the Great Depression, built the booming post-war economy, created plenty of millionaires, and developed a strong, affluent middle class.

Now conservatives are calling for a flat tax. There is no question that a flat tax would be much simpler, but is such a tax system really going to build a stronger America or just a wealthier elite, moneyed class. The following nations currently use a flat tax: Albania, Bosnia and Herzegovina. Bulgaria, Czech Republic, Estonia, Georgia, Guernsey, Kazakhstan, Iceland, Iraq, Jersey, Kyrgyzstan, Latvia, Lithuania, Macedonia, Mongolia, Montenegro, Mauritius, Romania, Russia, Serbia, Slovakia, Ukraine. Are they the models we should follow for a better future USA?