Hopefully we can strike a balance between leaving only your direct descendants better off and leaving everyone in the next generation better off.
> I would always be hesitant to tax money that in theory was already taxed when earned
This is incompatible with having an income tax and any other tax. Although, income tax isn't exactly well loved by economists, so maybe there is something to that.
You're thinking that taxing the wealth of a billionaire is guaranteed upon death. Therefore from your perspective there are two options: taxing the wealth or not taxing the wealth. Except taxation is the exception, not the rule.
If you know your wealth will disappear then you will try to get rid of it before it can disappear. With the estate tax there is a strong incentive to just hand over your wealth to your children before you die or by spending it all on donations so the government can't take it.
I'm not sure where I stand on a wealth tax, but I would always be hesitant to tax money that in theory was already taxed when earned. I could certainly see the idea becoming more acceptable if the importance of community and a traditional, tight-knit family structure continues to erode.