Earlier quoted context omitted.
The concept of marginal utility says that taking different amounts from people with different levels of income/wealth is treating them the same. To a person who earns $10k/yr, $1k is huge (whether it is extra income or owed tax). To a person who earns $1M/yr, $1k is basically pocket change. The utility of that marginal $1k varies for them both. Ergo, to treat them equally, we tax them differently.
> The concept of marginal utility says that taking different amounts from people with different levels of income/wealth is treating them the same. Marginal utility says that—most of the time–the utility of one more of an item to the owner is less than or equal to the utility of each item they already possess. (The exception would be where you need multiple items to do anything useful, but this is generally treated as…
I think this is nonsense. We can't put a precise worth on it for different people, sure. But I think it's indisputable that there's a curve, we know it's rough shape, and that it's a reflection of marginal utility. The lack of labels or positions on the axes doesn't make that graph useless.
> The concept of marginal utility says nothing about the relative utility of the same amount of money to different people.
Sorry, but this is just completely wrong. Here's the most relevant paragraph from: https://commons.lib.niu.edu/bitstream/handle/10843/22650/12-...
>The linchpin of The Uneasy Case is its rejection of the principle that income has diminishing marginal utility.' Diminishing marginal utility of income (DMUI) means that the greater a taxpayer's income, the less an additional dollar of income is worth to him. If DMUI holds, the government exacts a lesser sacrifice from a higher-income taxpayer, with each dollar taxed, than from a lower-income taxpayer. Moreover, any redistribution of income from a higher-income tax- payer to a lower-income taxpayer tends to increase aggregate welfare: The lower-income taxpayer derives greater utility from each dollar gained than 4 the higher-income taxpayer derives from each dollar surrendered.
Now, to be fair, there are arguments against the DMUI case for progressive taxation. For example, here's Donald Boudreaux writing for the American Institute of Economic Research:
https://www.aier.org/article/rich-man-poor-man-comparing-the...
The fact that it's a terrible argument doesn't diminish the fact that it clearly recognizes that there is a case to be made for progressive taxation based on DMUI.
Here's another one, equally bad:
https://www.econlib.org/archives/2011/04/the_uneasy_case.htm...
The key point here seems to be that even though DMUI is true, we don't know the actual numbers, and thus we should not act on it.
As for your remarks about $1M in 1yr vs $100k for 10yrs ... well, sure the tax code is necessarily imperfect. Optimize it for people with one income patterns and you've made it worse for people with a different income pattern. Such is life ... perfection is not an option here.