I think it would mostly balance out though, for individuals. Right now, their greed is boundless because they keep practically all profit made so nothing mitigates their desire for more, more, more.

But when they know they only get to keep 8% of what they make beyond a certain point, they won't have as much motivation to keep squeezing more and more out of their customers (or whatever the money source is). As the tax rate increases with income, an equilibrium would be approached where they wouldn't consider it worth the effort to keep pushing for more, knowing there's a functional limit to just how much wealth they can accumulate after that point.

I agree with you that this only addresses part of the problem. Another important legislative step is to take back control over corporations and forbid monopolies again! We used to, but the antitrust and other laws controlling their growth and power have been effectivley dismantled and now we're in a new gilded age.

Those two things could be implemented quickly, along with eliminating some of the more egregious tax loopholes. Then over time reform (or replace) the whole tax code (enormous task).