The parent is talking about "progressive" as in "the tax gets progressively higher as you make more money", not in the "social justice" sense. The flat tax being regressive means that the poor would have to actually pay more in taxes than they do now, which is very little. That's seen as a bad thing. Obviously there are solutions to that in flat tax proposals.
Second, "the wealthy pay very little taxes" is a meme. I mean, look at what we've got here:
http://www.pewresearch.org/fact-tank/2016/04/13/high-income-...
Those making $250K and up are responsible for 51.6% of all income tax revenue. How much more should they be expected to contribute? Remember: the truly wealthy can live anywhere in the world. There's a balancing act that needs to be established between "you're not paying your fair share" and "soak the rich."
And this is what is so annoying about groups complaints about the wealthy paying taxes. The heavy reliance on misleading statistics.
The quote says a specific group pays 51% of all tax revenue and says it's way too much. Well did they collect 51% of all income?
An entire article that not once answers the most obvious question even though they clearly have the data to answer it.
And to be clear, I'm not talking about Adjusted Gross Income (which is the total after taking every possible deduction in the book), but actually gross personal income. The latter usually isn't reported. The former is which often is used to make percentages of taxes paid seem higher.
And I don't know the answer, it may even help their case - but the fact the most obvious stat question isn't brought up in arguments for why taxes on the wealthy are too high makes it easy to conclude these aren't discussions in good faith.
Did they get 51% of all the benefits of how that money was spent?
Probably not. Some quick Googling finds this analysis:
http://www.financialsamurai.com/how-much-money-do-the-top-in...
It uses different split points, but claims that the top 5% of earners make about 35% of the total AGI, but pay 59% of gross income taxes.
The IRS is so nice as to provide exactly the metric we're looking for, which is:
For the group of people making $XXX per year or more, what percent of all income in the US does this group earn, and what percent of all income tax does this group pay?
Here's the groups you mentioned:
+------------------+------------------------+---------------------+--------------------------+
| Income group | Percent of tax returns | Percent of total US | Percent of total US |
| | in this group | taxable income | Income Tax owed |
| | | earned by group | by group (after credits) |
+------------------+------------------------+---------------------+--------------------------+
| $200,000 or more | 4.20% | 41.90% | 57.50% |
+------------------+------------------------+---------------------+--------------------------+
| $100,000 or more | 16.00% | 67.90% | 79.50% |
+------------------+------------------------+---------------------+--------------------------+
It is a bad thing. In low-income households, there is less disposable income. In that case, taxes eat into the cost of necessities (or near-necessities such as broadband for those with slightly higher income). On the other end of the spectrum, for those with large incomes that income is largely disposable.
Dollars are dollars, sure, but there is a distinction to be made between taxing someone's food money and someone's Rolls Royce #2 money.
As an American who often lives anywhere in the world, I'd like to point out you pay US taxes no matter where you live. (though you do get a 90k income exemption, if you are talking about the "truly rich" then that doesn't matter a whole lot)
Not to mention that there is a huge misunderstanding of how progressive tax rates work. A very affluent businessman was on tv saying they would essentially have no motivation to work if the marginal tax rate increased for him when Obama was campaigning. His idea was that the tax bracket above 250k per year would somehow affect his entire income if he earned more than that figure so he would only want to work until he had 249,999 dollars and then stop making money to avoid "making less money". Of course the marginal tax rate is the rate that your money is taxed above that threshold. In this case the tax rate would go from say 15% to 20% on the money earned after 250k. Let's say you made 260k that year. Your extra tax burden because of the marginal rate increasing would be 20-15= 5% on the 10k dollars. You don't get taxed an extra 5% on all your money made like that fellow thought.
Flat tax is regressive and sales tax is regressive.
Due to this, I will focus on what it means to replace the income tax system with a flat 20%. A family of 4 (2 kids under 18) making $50,000 per year pays about $200 in federal income taxes if they do nothing but file their taxes and take the standard deduction, personal deduction, EITC, and Child Tax Credit. A family of 4 making 70k pays something like $2000. A family of 4 making $150k last year contributing to 401k, HSA, IRA's, 2 kids under 18, have an effective tax rate of like 8%.
In every single one of these cases, the families would be worst off (some significantly) if they were required to pay a 20% flat tax.
Increasing taxes on them would hardly qualify as hardship. The utility of a dollar is much lower when you have ~$300,000 in savings (80th percentile), then when you have ~$300 in savings (20th percentile).
Related anecdote: With an annual income of ~$250-300,000, my net tax rate, including untaxable benefits and investment income is the same as that of my partner - who makes ~$30-40,000. If I had an income of $2,500,000, it would be substantially lower.
[1] https://en.wikipedia.org/wiki/Wealth_inequality_in_the_Unite...