> And similar proposals like the Negative Income Tax would cost far less money and have none of the presented downsides.
It all depends on how you tweak the numbers; in theory a negative income tax and a guaranteed income cost exactly the same amount. A guaranteed income of $1200 taxed at a marginal rate of 50% is just the same as a marginal tax rate of -50% on an income of $400. That being said, there are some pretty big negative externalities to a negative income tax, in the sense that it even further overburdens the tax system with knowing people's exact monthly income (assuming monthly payments), which is not-at-all straightforward for the poorest taxpayers whom presumably such a system would be designed to most help.
They are mathematically the same, depending on the tax curves.
A negative income tax doesn't mean you get -50% of $400, it means your income starts negative. So someone making $0 gets like $1000 back (say by paying 20% over -$5000).