Under such a system — unless I've badly misunderstood something — if a billionaire's assets decreased in value over the course of a year, they would still pay 2% on their assets. I can't think of any sense in which a decrease in the value of one's assets would be defined as income.
I have an opinion of the wisdom of a wealth tax, and I could be wrong. Regardless of my opinion, I think it's indisputable that a wealth tax and an income tax are different and that conflating the two makes a debate on the merits much more difficult.