My guess is that private businesses of this size would likely be able to move enough of their funds from capital gains to dividends to cover any expected taxes needed. So any company that had a 20% CG increase could instead divert 4-5% as a dividend in order for the owners to cover needed taxes.
@FIPurpose The above statement is incorrect.
Here's an example that shows how the mark to market ("MTM") accounting works.
Some top one percent person owns a $1M house, holds $2M in retirement accounts, and owns a business worth $1M.
Because taxpayer and spouse are in the top percent, MTM applies.
Applying the MTM math, if the business increases in value by $1M (say because its profits grow from $150K a year to $300K a year), the taxpayers owe $400K in income taxes due to the MTM. (Assuming 40% tax rate.)
Note that taxpayer's actual cash income increased by $150K but the taxes equal $400K. That's why Warren has to let them pay the taxes (with interest of course) over five years.
BTW, if the retirement accounts increase in value by $1M, no MTM triggered taxes. Retirement accounts excluded from MTM accounting.
Also, if the house increases in value by $1M and that increase would be treated as capital gains, the MTM adjustment is also $400K.
P.S. The capitalization factor, essentially a price earnings multiple, that Saez and Zucman used in their research for small businesses was 6.7.
P.P.S. Income on house appreciation that can't be sheltered by Section 121 should be subject to MTM too.
P.P.P.S. That link below to my blog lets you get to the most recent post. It discusses this stuff in detail and provides a bunch of examples.