Part of the reason the tax code is so complex is the goal of a progressive tax system where wealthier people pay a higher share of their income. A flat tax, while simpler, is regressive- everyone pays the same percentage. Couple a flat tax with an unconditional lump sum grant, however, and you get both progressivity and simplicity.

> Part of the reason the tax code is so complex is the goal of a progressive tax system where wealthier people pay a higher share of their income.

This is false.

Almost none of the complexity of the tax code comes from this. The complexity comes from counting up how much income you have. After that it's a trivial calculation to determine your tax owed.

Yes, if you change to a flat tax when you go to UBI. It would make sense, but... Congress.