Your heirs didn't earn that money, so they're only paying taxes once. You pay once for earning it, they pay once for "earning" it via your departure from this earth. But presumably if you have lots of money, you've already set up trusts, paid for college, given lifetime gifts, etc. so your heirs are already pretty far ahead of the average person.

Seems like a perfectly good way to encourage at least a tiny amount of wealth redistribution in a society where wealth is not very well distributed at all.