So you argue that the price of a Peppermint Mocha Vente should be progressive; subject to the status of automobile you pull up to the drive thru window in?
You recommend a form of commerce in which shoppers present their bank statement to the Kroger cashier prior to checking out so proper adjustments can be made to their cart of groceries and sundries?
In the name of fairness of course.
It's as if you're purposely dense.
Let's walk through this step-by-step and I'll use the average numbers here: How The Poor, The Middle Class And The Rich Spend Their Money : Planet Money : NPR which outline how Americans (low/middle/upper class) spend their money.
Say that the state imposes a total 20% income tax (aggregate % accounting for both state and federal taxes). What does that mean for a poor family (defined as having household income of <$20K? It means that they are paying $4K in taxes. According to the data above, such a family also spends an aggregate of 14.9% of its income on food (groceries + restaurants) for a total of $2980 per year and 29.2% of its income on housing for a total of $5840 per year and 11.1% of its income on utilities for a total of $2220 per year. So if we just look at the NECESSITIES OF LIFE, meaning shelter and food and add the mandatory flat tax to it, this family is left with a grand total of $4960 for the whole year. With that, they need to pay for gas and transportation, health and medical expenses, school expenses for any children they may have, daycare, entertainment, CLOTHING (which should be a necessity but I didn't want to count it in case you were going to argue that they spend disproportionately on nice clothes), etc. Nevermind that I didn't even account for consumption taxes which would be applied on items such as food and clothing.
Now take somebody like me. With our household income, if you proposed a 20% flat tax, I would pay about HALF in income tax as I pay now. Practically speaking we are talking about some $80-90K less in taxes than I pay now. Great! Sounds like a deal. And you know what we'd do with that? Not open a business or hire people or go out and buy, buy, buy (George W. Bush's idea of how to battle a shitty economy). We'd put it in the stock market, utilize the even lower capital gains tax rate and get off like bandits.
So when the system becomes untenable, as ours will shortly and as yours would even more quickly, then we start to talk about cutting the social safety net and raising the retirement age. From 65 to 67, maybe to 70, 72, 75. We can't afford to have all these people retired and relying on benefits, and now that you've cut revenues from the upper classes and the government is broke, you need the people to keep working, longer and longer. And guess who doesn't give a shit about the retirement age being raised? People in the upper class whose taxes you just cut, who got away like bandits for decades and who are going to have their feet up at 60 and not standing hunched over working as greeters in Walmart until they're so decrepit that we take mercy on them.
The flat tax idea is supremely inequitable because it is not about who pays the most in TOTAL taxes, it is about how individuals allocate and spend their income and what proportion of one's income is going to get eaten up by taxes. You can arguably make a flat tax rate society more equitable by allowing significant deductions and exemptions to the poor so that they have $ returned to them at the end of the year and the rest of us do not. But I bet that you have no interest in that structure.
If I was a really, really selfish person, I'd love to live in your world.