The estate tax, or "death tax" as its critics call it, is an essential aspect of the American republic. Indeed, it's an essential aspect of any republic. Recall that essential means something can't exist without whatever is that essential thing, which is exactly what I mean; a republic cannot exist for long without a meaningful estate tax.
Several premises of the American republic were: that the aristocracies of Europe were not serving the interests of the vast majority of the people, and that the people have inherent rights that are not being recognized by the ruling class of aristocrats. That the people were capable of governing themselves and only through this self government were the people able to ensure that their rights were defended.
Now what exactly is an aristocracy? To the casual observer it appears that it is a system of interrelated families that have members who have titles which grant them a certain status. And that the members inherit these titles at birth, so you have to be born into the system. That's a pretty closed system. These are all true, but are really not the foundation or the causes of the aristocracy, rather these are the symptoms.
An aristocracy is primarily a system of inherited wealth. Wealth brings power and influence, so that those with great wealth have much of both, and when they band together their share of both grows markedly. This group, by necessity, is a very small fraction of the population, perhaps less than 0.01%. This very small group wields enormous control over the lives of the rest of the population. This group may not act in the best interests of the majority of the population, so some rational for the legitimacy of their actions needs to be enacted. That mechanism is the idea of hereditary aristocracy. A system which cannot be breached from the outside due to its inherent closed nature. Thus, the population is led to believe that the right of the aristocracy to rule is natural and permanent. But of course it's not. Each title and ceremony and right were crafted by those in power to solidify their power, so much so that they appear to be an essential part of the the state.
What do the rest of the population think of this? Inevitably, the aristocracy will take actions that will disadvantage the rest of the population. Some of the people will recognize this and be unhappy with the situation. They will band together and take action when the opportunity arises to change the situation. The American revolution was just such an opportunity. The result was a republic "of the people, by the people, for the people". The challenge then is to prevent the formation of a new aristocracy that can overturn the republic.
The key to preventing the formation of a new aristocracy is to prevent the passing down of inherited wealth from generation to generation. That is, to prevent the creation of what people rightly call dynasties. The very word dynasty refers to an aristocratic structure. So what is the best way to prevent the passing of great wealth and the formation of dynasties? The inheritance or estate tax.
This single essential need, of preserving the republic, is rationale enough to defend the estate tax from the endless challenges of those who think it unfair, or big government, or punishing success, or whatever other argument du jur.
So, I repeat, a meaningful estate tax that prevents the passing of great fortunes from generation to generation is an essential aspect of a republic.