Stepped-up basis was invented back when estates of much less than $12 million were taxed.  Eliminating it would mean more tax collected from the wealthy, which could allow for millions with low income to be removed from the federal income tax rolls.  Did you see the story today about how Jeff Bezos qualified for child tax credit a few years ago because of paper losses reducing his AGI to a loss?  You can't make the impact on the average taxpayer much more negative than it is now.