I don't really disagree with you. But I've done the math and when you compare a deficit neutral flat tax plus ubi (flat tax would need to be around 35-40%) to a deficit neutral progressive tax maximizing the average take home income slope (ie. making it so people experience some benefit to seeking higher raw income and avoiding poverty traps) the difference is miniscule and not worth the effort to implement.
Tax on accumulated wealth is where it's at beyond flat tax + UBI.
