Money is a claim on future production, and you don't get to transfer that claim over other people's work without caveats, whether it's inheritance tax, limits on gifts, or wealth taxes.
So how about $5 mil? Nice larger lot home in Palo Alto that some dying parents bought in 1940. Not even retirement money in the bay but could be middle class retirement in Alaska.
Where is your “obvious” line? Your handshake/battery analogy is shitty because it involves a victim and medical/first responder reports.
We’re talking about setting some line where we feel as a society it’s morally ok to start taking money that has already been taxed strictly based on “that seems excessive” without any logical criteria.