Imagine a person was abused for a large part of their childhood and is subsequently traumatised and mentally ill, then...

Withholding an inheritance from someone because you abused him and he dared to take you to court is unethical. So this "extortion" is being used as the only way to get compensation while avoiding being the victim of further unethical behavior.

So no, it's not unconscionable, although whether it's legally extortion would require asking a lawyer.