Most inheritance is dissipated in something like four years.
If someone is clever enough to create a huge wealth that could withstand the typical degeneracy of heirs, then they are very likely to do like the Rockefellers, and create a structure designed to weather the changes of tax law.
There are very very few Rockefeller-class families.
IOW I think the point of a law to confiscate heir's wealth would be wasted, besides being a transparent confiscation of wealth that the government has no actual title to. Do you see wealth as a static thing that 'the rich' pile up in a little dragon cave?
Ed