> A utilitarian could (and has) claimed that this marginal dollar occurs at the point where you have the bare minimum to survive
There's a few problems with this line of reasoning:
1. Its not clear to me that this argument is being made by a utilitarian, as you claim
2. You at least appear to show a misunderstanding of what "the marginal dollar" is. Every dollar is the marginal dollar. Its just that the relative value of dollar 1 (which you need to survive) is greater than the relative value of dollar 1 Billion, which you use to, uhhh, buy an island. I think what you're saying is that there is a point where the marginal value of your dollars is low enough that you are ethically obligated to donate them is at or around subsistence level, and that might be true, but it leads to the third problem:
3. Even for a strict utilitarian, if you take into account externalities, the right thing to do might not be to take all the money away from people. If you're optimizing for something like reducing death, reducing everyone in the US to subsistence level probably isn't the best solution: you'll likely do something unwarranted like cause a revolt. This has two major net negatives: a bunch of people die in the revolt, and you end up losing a lot of your money, either because you spend it to put down the revolt or because you don't and you lose and people take it back.
4. While improving the life of those in absolute poverty is a laudable goal, it's not clear that it makes sense for a government to do that, except when it ends up being beneficial to the government or its people. In other words, a government's moral imperative probably doesn't extend (far) beyond its borders.
> So I don't think a strictly utilitarian argument is sufficient.
People often forget about externalities.
On the other hand, if you think of the supply of billionaires as a revenue source, and then you think of there being decreasing demand for being a billionaire as taxes go up, but increased revenue per billionaire as taxes go up, you can draw up supply and demand curves and optimize to maximize revenue from billionaires (or people in general really, its just that the marginal rates can likely be higher on billionaires and they're the talking point of the day). If there are 100 billionaires, and you tax 2% more, and 1 billionaire moves away, you're still up on revenue. I often hear people arguing that we should operate our government more like a business.