That massively massively penalizes people who are more poor or middle class vs someone that has millions or billions. 30% of a million is 333k, but 30% of 31k is 10.3k. I imagine losing that 10.3k hurts the 30k person far more than losing the 333k does to the person who is now making 667k.
Why would you want a flat tax for everyone above 30k?