The point is not the amount of revenue. The point is that having billion-dollar inheritances creates incredibly skewed political incentives which end up leading to significant harms.
If they taxed the estates at a high marginal rate with no gaping loopholes, auctioned off all the assets thereby recovered by the government, and then just zeroed out the bits representing the dollars in the resulting accounts, that would be just fine with me.
Ultra ultra wealthy people are not being “punished” by having their estates taxed at a steep top marginal rate. The great-great-grandchildren of industry titans do not have a preordained right to massive wealth at the whole society’s expense. Money is a social construct, and can and should be designed to work for all of us.
In a functioning society with a growing (or even stagnant) economy, each generation should inherit a greater wealth of public infrastructure, capital, services, knowledge, ... than the previous one. Instead, in our society there is an incentive for public officials to transfer collective assets to individual aristocrats in corrupt deals, to underfund/destroy public institutions, to take out loans guaranteed by the state and pocket the money, etc., impoverishing the next generation. This amounts to basically theft/fraud against the polis.