I understand your sentiment. But the highest marginal estate tax rate is currently 40%, for amounts above $1M (after deducting applicable exemptions). That seems high enough. The problem is that the estate tax system is so complex, so full of loopholes and tricks, that no truly rich person with 2 brain cells and a competent estate lawyer will pay a fraction of that 40% tax.
I don't know what the solution is, but it seems to me that the estate tax system is overly complex, too open to abuse, and needs to be simplified. It is unfortunately intertwined with Trust laws, Probate laws, Income tax laws, international tax treaties, etc. As far as I can tell, the estate tax exists only because of the "stepped up cost basis" feature in the US tax code, where all capital gains is zeroed out upon death by increasing the cost basis of the asset to the current market value. (Does any other country have this?)
I think it would be simpler to eliminate Stepped Up Cost Basis, and replace the estate tax with something like a "deemed disposition" tax, where the capital gains tax is paid upon death (above a certain exemption amount, say $5M/couple), or paid upon transfer to another non-pass-through legal entity (e.g. an irrevocable trust). Oh, we should also eliminate the long-term capital gains tax rate, and all capital gains should be taxed at the same rate as earned income. I think this would eliminate the "carried interest" loophole. Oh, and we should eliminate Dynasty Trusts, which can last as long as 365 years. It's hard to see how allowing a Trust to last 365 years is good public policy. Oh, and so many other loopholes need to be closed.
Unfortunately, I don't think there is much political will to tackle these issues. Estate tax law is a very obscure part of the tax code. Very few people care, except for the small number of ultra wealthy people and their lawyers who are taking full advantage of the current system.