If you follow the History of the Estate Tax, since 1797, it rears its head after every war. All the way through WW2, before it became a "permanent". Fixture in the Tax Code.
Thomas Jefferson suggest that those who benefit most from the free society have an obligation to pay for it''s defense. Anyone disagree with that?
In principle perhaps, but you seem to have determined that the only, or at least the best, way to determine the extent of benefit someone has derived from living in a free society is the extent of their wealth.
A person who has come to this country to avoid the high probability that they will be killed in their home country for their political or religious views, has obtained an enormous benefit, that, in most people's minds, exceeds great wealth in actual value.
Bill Gates has, obviously amassed quite a fortune in our free society, but there is little actual reason to believe he could not have done the same in a much less free society. How much of his wealth is attributable to the fact that he developed it in a free society. There are billionaires around the globe and that includes countries with far individual freedom than ours: China is #2 (behind the US and ahead of Russia) on the list of number of billionaires.
Someone living in the bottom 10% of our economy have a much better lifestyle than some their counterparts in China or the Congo or any number of other countries where society is less free than in the US. Does this mean they have benefited financially from living in a free society? And if the difference in personal lifestyle between them and their Chinese counterpart is much greater than it is between counterparts at the highest economic strata, can their benefit be said to be greater than that of the billionaire's.
In any case, no one is arguing that the wealthy should not be taxed, nor do I believe I have seen an argument made that a progressive tax policy is unacceptable.
You've remarked that an Estate Tax of exemption of $1.5M per person is OK, but $10M is not. On what to you base these judgments? You personal sense of how much rich people should be allowed to leave their kids? It's OK to make them wealthy when they die, just not very wealthy?
Is this a matter of "the government needs more money so the wealthy should provide it," or "no one should be so wealthy, and the government's the only one that can 'legaly' take their money"?
Are you content with the way the government now spends the money they take from us? Are you confident that if they get more from increasing the Estate Tax they will put it to good use?
Should they take only what they need, or should they take as much as they can and then figure out what to spend it on?
Do you think that if they take more through the Estate Tax they will
a) Not increase what they take from you
b) Take less from you.