I’ve used the word imaginary so I think I might be to blame. But I am not in any way arguing for the rest of your sentence.
The real world issue is not whether value can be estimated. Obviously there are specific times where it can, as the examples for stocks and houses show.
Other instances are less easy. The wealthy may own five houses and pay property taxes on those. Chicken feed for the rich. Much more of their money lies in the ownership of businesses. When bringing a suit, the feds often spend years tracing down companies owned by companies owned by companies which aren’t based in the U.S. and don’t have to reveal anything. Piketty needed to resort to an imaginary world of global transparency of all ownership to get around this limitation.
We know, supposedly, that Musk owns, say, $100 billion in Tesla stock. Does he? Or does he have a corporate structure that owns the stock for him, meaning that the corporation pays corporate taxes. Even the ordinary millionaires like doctors and lawyers and bestselling authors have either a PC (Personal Corporation) or LLC (Limited Liability Corporation) that effectively treat them like employees and reduce their taxes. Billionaires must have superdeluxeultra versions of these.
From here on in a tax lawyer is needed, but corporations and people are normally treated differently for tax purposes (there are some exceptions for one person corporations). Loans are liabilities and can be put against income just as capital losses can be put against capital gains. Some corporations are known to deliberately acquire losing firms just to be able write off income from the successful ones.
Most of that is murk to me, and to most governments who struggle every year with trying to handle large businesses and keep winding up with headlines about Fortune 500 companies paying no taxes in a calendar year. They have better and far more numerous lawyers and accountants.
Smaller governments retreat to ad valorum taxes like property taxes. Those have the advantage of being assessed and being fairly stable over time. California - uniquely, I believe - keeps the initial assessed value of a house until the house is sold. If OlderOldOlds’ house is in California he will pay taxes on a $44,000 house. If he sells the new owner will immediately pay taxes on a million dollar house, another reason why the housing market is so terrible there.
The fact that a bank would presumably tender a second mortgage/home equity loan for $500,000 is an incidental way the system works. Value can be treated as collateral because most of the time the system works. The giant crash of 2008 happened primarily because unreal value (bonds on mortgages unlikely to be paid back) was peddled as top AAA quality value, virtually guaranteeing their safety, and failing spectacularly.
All this stuff way down in the weeds is what allows the wealthy to handle their wealth differently from the ways ordinary workers handle their income. Many, many changes can be made in the system to reduce the advantages of the wealthy, but they are systemic changes that each and severally as the lawyers say will be fiercely fought and have unintended consequences that may hurt others. Any poke at a system this complex and this badly understood spreads through links unimagined even by the best at the beginning.
No such thing as a simple wealth tax exists. Potential - is that a better word than imaginary? - money is not the same as spent money, although people can and do treat it as such in special specific cases.
And where’s the political will to make any changes at all, no matter how small?
Their first step will be attack the IRS and further hurt its ability to go after the wealthy. Changes to that system are imaginary squared.