Why can’t it be both? Could have a logarithmic scale, could have it increase by a fraction of a % of $(value - currentTax) so that it increases more slowly over time.

Not saying I agree with the underlying idea, just confused about why “increases ever month” necessarily means it cannot be “tied to the value of the property” in a `calculateTax(propValue, timeEmpty)` sort of way.

So over time the vacancy tax is increasing and the sale price is going down. Obviously the vendor needs to sell or redevelop before the price hits zero. If they fail to do so then they lose the property. Speculation sometimes fails. What's the problem?

Good, then it will go to someone who needs a house, not another rent-seeking parasite.

That does not seem like a realistic problem for very many places. If a jurisdiction contains land with so little value that it won't support the taxes required for upkeep of the jurisdiction, then the jurisdiction could dissolve. Which is what happens to a ghost town I would imagine. That is not likely to happen to New York City.