Progressive proposal for wealth taxes--why not just do this?

Property is a tangible thing that creates a continuous flow of services. A property tax is essentially a tax on the value of that flow of services that the property creates, kind of like a sales tax when purchasing a good.

Taxing paper wealth, just for existing, is something else entirely.

It is a spectacularly bad idea for reasons that are difficult to explain in brief. It represents a thorough misunderstanding of what taxation actually is.