Especially since there are countries that do tax at least some capital gains at the same rate, and they don't seem to be doing appreciably worse than other countries in a similar economic bucket that do not. Indeed, it seems to be more popular among economically liberal countries with flat income taxes.
Perhaps, but the US is not past that point. Every level of income pays a higher rate of taxes than the level below it according to IRS reports.
Separately, none of the issues I listed are hypothetical.
Does their definition of "income" include capital gains for the purpose of those reports? I don't see how this could possibly be true if it does, given that capital gains constitute the largest part of overall income past a certain point on the scale.
I'm not sure where it is today, exactly. But it's easy to see that it's always going to be there, simply by virtue of increasing proportion of capital gains at higher income levels.