That’s a non argument. The VAST majority of wealth we are talking about is in equities which absolutely have a clear price. All that other stuff is noise to confuse the point. You really think the wealthy will put all their money in art that has no intrinsic value? And wait a sec, even if they did, what did they just pay for it? That seems like a perfectly valid basis for tax.
None of these values are ever perfect. The point is, we pay a small percentage of our assets every year, but the wealthy have convinced us they couldn’t possibly. Most don’t even “own” anything, it’s all under tax and liability protected companies.
But for the most part, patients aren’t really their customers. Employers are. They may want to decrease premiums, but making changes is difficult and at most an annual event. It is very, very far from a free market in the US.