Billionaires looking to pass on their wealth to future generations can avoid capital gains tax altogether by holding their assets until death, Joshua Wu says. The heirs receive the assets with a "step up in basis" — instead of paying capital gains tax based on the original value of the assets, they pay only if the asset appreciates beyond its value at the time of their benefactor's death.

Furthermore, the 2017 tax law doubled the amount excluded from federal estate tax to $11,180,000 for individuals and $22 million for married couples, a hefty boost for America's richest families. And the amount has been edging up since then, too.