If you're making millions of dollars a year you're probably only spending a small fraction of it and if there's only sales tax you pay tax on a percentage of what you spent. So let's say you made 30 million dollars and spent 8 and say the sales tax rate is 15%. In this case you paid 1.2MM in tax for a marginal tax rate of 4%.
On the other hand if your income is 15k you're probably spending it all or very close. In which case your marginal tax rate is 15%.
Not all that fair I'd say.
Personally I like the progressive income tax method. I just think capital gains should be taxed as income rather than at a flat 13% and no special rules for corporations either. If they want to be people they can be taxed like people. If they want to be different then we can just get rid of corporate personhood. I doubt they'd like that much.