I think you're arguing that the financial instrument itself can't be $0 and because of that there's no way for valuation of an entire company to be less than $0

I think you're arguing that the financial instrument itself can't be $0 and because of that there's no way for valuation of an entire company to be less than $0. I never argued anything that means that the financial instrument of a stock should be zero or less than zero. I think a company stops being traded entirely if it should fall to zero and enters into bankruptcy to pay off existing creditors. Maybe then stock is converted into something else, I have no recollection of any instrument that represents that owning a stock means you are indebted now. People do go after the stockholder if the holder was trading on margins and the margin call happens from the broker to recoup their losses. But this scenario is really between a broker and a client, not shareholder v. anyone else relationship.