Aren’t you omitting something here? Like the fact that the step up in basis happens after the inheritance tax is paid?

If I buy stock for $10 million and in 40 years it appreciates to $70 million (an average 5% annual growth), and then I die, my kids will pay inheritance tax on the $60 million capital gain. Then the new basis will become $70 million. What exactly do you propose it to be?