A 20% increase in marginal rates is definitely going to affect the incentive to work. Middle class wage earners will be paying 50% to 60% of their income.
If you want to save on the cost by scrapping existing benefits, the $12,000 a year would probably not fully replace the social security pension, disability benefits, or EITC/food stamp/TANF benefits people get, or at best be approximately the same. So you haven't changed things much for people at the bottom, in exchange for a massive tax hike.
A massive tax hike combined with a simplification in government should result in larger revenues. If things don't change for people at the bottom, who did they change for - all that extra money has to go somewhere, holding all other government spending constant. Presumably the lower middle classes ended up the recipients of this massive tax hike, in your perspective. But this is just a function of the progressive tax curve; it and the fixed payment can both be adjusted to achieve a particular redistribution.
The more interesting thing, to me, about an unconditional fixed income, is that it changes the nature of work at the margin. Benefits don't turn into poverty traps if they're unconditional. Incremental part-time work doesn't come hand in hand with the risk of losing your benefits. I think employment overall, and productivity, would increase.
Pensioners/disabled/people receiving unemployment benefits would be worse off.
The big winners would be people who are not working and not receiving benefits today, and presumably the people who live in the same household as them.
One way of modelling basic income is as a negative income tax.
However one of the advantages of making it "unconditional" (but not necessarily actually beneficial to high income people) is that it buys political acceptance from the middle classes especially.