Except this was true during WW2, also. You have to do an apples-to-apples comparison of what the top 1% of income earners paid then vs now. The shift in income to capital gains tax doesn't matter since the capital gains tax is the same, and the shift would have simply happened quicker with income taxed the way it was.
Also, collateralized loans don't change anything besides simply delaying when the tax is paid. One needs to realize some gain somewhere to be able to pay back the loan eventually.
> The data in your source indeed does not seem to consider capital gains tax, as they seem to reflect the top tax bracket on average for different states and locales, whereas if capital gains were considered in relation to the state in which they are realized, it would certainly be much lower.
Yes, but capital gains taxes have been the same, so in effect this source undercounts the intensity of this effect. It doesn't matter if there are more multi-millionaires today that do not qualify as the "top 1% of income earners", because they would pay the same capital gains tax then vs now.
The data in your source indeed does not seem to consider capital gains tax, as they seem to reflect the top tax bracket on average for different states and locales, whereas if capital gains were considered in relation to the state in which they are realized, it would certainly be much lower.