The estate tax is taken, and used only for the sole purpose of Medicare and Social Security. Under those strict circumstances, I would support not just an estate tax, but a large one.

Here's my thought process:

There are a very large number of retired people out there living a very good life. They, my parents included, have amassed significant wealth through their life, and with the benefit of SS and Medicare, they are able to live like kings without eroding their life savings very much. In fact, they are able largely to just live off the earnings without spending much principal.

Good for them, however, with this system being headed for insolvency, it doesn't make sense not to address this issue. But if you do means testing, all you do is provide incentive to hide wealth and/or transfer it sooner in order to get in under the wire to qualify. That introduces a whole host of unintended consequences.

If you instead, do as I suggest, then A) they don't have to worry about it, since they are dead. B ) they know that it will go to maintain the system they benefited from. C) it will go to sustain the system for their heirs (just as it would had it been handed down albeit less directly).

Seems to me to be a fairly moral argument, and one that could be popular with almost every demographic.