But the flat tax with a prebate (essentially a variation of fairtax.org) *IS* progressive. With the plan OP proposes, everyone gets $1000/month. Enough to live (depending on area), but not enough for most to not want to do better. You're getting $1,000/month or $12,000/year. At the hypothetical 25% you would have to have an income of $48,000 before your NET tax rate was even 0%. 25% becomes the upper limit of the very wealthy. A $250,000 income pays $62,500 in taxes - $12,000 prebate = $50,500 OR 20%. A person earning $75 pays $18,750 - $12,000 prebate = $6,750 or 9%. Below $48,000 you're essentially getting tax credits / welfare / scholarship / whatever you want to consider it as. It's yours to just stay out of the way and/or invest in yourself.
I know there are problems with this approach too, but it seems like a fairly good approach to me, and it completely eliminates the holes that people constantly encounter now. For instance, I know people that have deliberately sought pay cuts because their last promotion put them over the income limit and they lost financial aid for children / medical / etc. Now, despite bettering themselves and being more productive, they have taken a net loss in income that they cannot sustain. I would also rather unemployment / welfare / etc not punish people for finding work. A prior neighbor did everything in her power to avoid getting hired. Her unemployment required her to look for work, but her unemployment paid better than any jobs in the area during the recession, and was more dependable (most jobs were part time / seasonal around that time as we'd just had a big manufacturing plant shut down and flood the market with workers). As a tax payer, I'd rather her take a job at BK and keep *half* her unemployment, and we'd both win. She'd take less of my tax money AND she'd have more money at the end of the day.
This also prevents the situation of it not being worth it to work. Yes, you already have a minimal lifestyle provided for nothing, and I know people that literally want or need nothing more. But even a minimum wage, part time job that plays, for fun math, $12,000/year, still gives you a $9,000 year gain over not having done anything (in the above scenario). So they now have $21,000/year. Yes, they are taking tax money, but if that's what we do as a society to provide for those that will never and can never do better than wiping tables at McDonalds so they have enough to hopefully not feel the need to mug us in a dark ally, I think everyone could walk away a winner. Automation will destroy the jobs at the bottom, and even if we provide education not everyone can do better, no matter what your second grade teacher said. We can't all be astronauts.