The point is to redistribute some of the money towards other hands and other goals, if you tax them just so that they keep the money flowing, they'll just keep it flowing to avoid the tax but the money won't change hands and goals: it'll still be in the hands of these billionaires and will accomplish the goals of these billionaires. Besides, it's easy to fake services in order to transfer money, that's how they operate tax heavens, so keeping the money flowing will be a piece of cake, even if the money doesn't do anything other than amassing interest while changing bank accounts every day.
The aim to tax on capital is to take some of the money away from those hands and goals so that public interests are served. Now of course for public interests to be served it requires a somewhat efficient and honest government, but that's another problem entirely.
My point is not that it's too much money. It's just a lot of money, and taxing it heavily is not a problem for the living standards of these few people, while it can make a tremendous difference in the lives of many, so that's not something I'm going to cry about.
I don't think heavy taxes on the rich will be as effective as it could be in current political systems, because it requires systems where power is a lot more decentralized so that it cannot as easily get corrupted, but it's definitely not something I'm going to cry about. Redistribution is necessary in order to have a society which treats its members well, and even though some of this money will be taken back by the extra rich through lobbying and corruption anyway, at least some of it will be used for public goods, which is better than nothing.
In any case, the "don't you see I'm going to be left with only billions" line by those rich people is asinine and certainly not worth my outrage.