Taxing unrealized gains will be extremely complex, and given that they aren't allowing us to deduct unrealized losses its a pretty shitty setup for the taxpayer.
We need to drastically simplify our tax code rather than further increase its complexity.
If you're a billionaire who does the "take out loans against your unrealized cap gains" trick, then you, you know... can't sell your stock. So then your stock passes to your kids -- who, due to the stepped up basis, yes, do not have to pay cap gains on that stock.
But there's a 40% estate tax.
Estate tax generally isn't very relevant even to the ordinarily-rich, because it has an extremely high deduction (about $27M for a married couple), but for a billionaire it's absolutely relevant.
Now, sure, if you paid both the cap gains and the estate tax you'd pay that much more taxes, but if you compare a normally-wealthy person (pays 15-20% cap gains and 0% estate tax) and a billionaire (pays 0% cap gains and 40% estate tax), it's obvious that the billionaire, eventually, pays a much higher tax rate.