WRT this: "I have three answers." I would add a fourth answer: the stupendously rich have a lot of assets in stock and so on that's not really about the money, per se, but about control. It doesn't seem like Zuckerberg, or Gates before him, deeply cared about having 1x billion + z billion; they cared about having control over Facebook and Microsoft, which happens in the form of stock ownership.

However, I don't see this as the motivating factor for the person Stross mentions in his last paragraph, so, to the extent the post is mostly about that paragraph, I don't have a great answer.