This is the logical fallacy in the anti flat-tax argument. You assume that having spare pocket money would empower upwards economic mobility, which I disagree to.
I would even go as far as to say they're entirely uncorrelated, since the sums were talking about are minor in the grand scheme of things.
Let's assume that as a middle class family you'd save 12k/year extra from XYZ's major super awesome give away freebies tax plan. If you're the average family, as you would imply, you're likely to spend part of that on frivolous and materialistic items, and the rest as investment towards retirement. Assuming however that you save this money for 20 years, we're talking 240k in flat savings (assuming your fund kept an even, ****** keel). A decent chunk no doubt, but oops, doing that bumped you into a higher bracket, and there's a higher tax rate on investments under XYZ mega super awesome tax plan. All those gains become rather marginal, and 240k is hardly a noteworthy transition to millionaire status. That's not to say that you can't make the most out of those investments, but those that do are likely moving forward due to other factors.
Rather, I contend that upward economic mobility most often correlates with these factors:
-Luck
-Hard work
-Education
-Access to capital (ala trusts, vc's, angel investors, etc)