But in practice much of the wealth of super-wealthy people is actually either tide up in the value of the businesses that they own, which are often doing economically valuable things, or is invested in useful enterprises (shares), or funds useful activities (bonds). It's not as though the net wealth of Warren Buffet is all being thrown on hookers and blow.
There are already ways to direct the spending of the wealthy towards more productive uses, such as consumption taxes on luxury goods. But if they take their wealth to other countries with laxer consumption taxes, there's no a lot we can do about it. So we're back to the libertarian argument. At some point you get into questions of freedom and individual rights.
The problem is when poor or middle class people are unable to improve their situation or lose ground.
The only time rich people are a problem for poor people is when rich people are able to corrupt government to tilt the playing field their way. This is a problem of corrupt politicians and lack of anti-corruption law.
I think people underestimate how many economic difficulties are not caused by economic effects, but by corrupt politicians who are permitted to stack the deck against the average person as a way to fund their campaigns or rack up post-governance favors.
So, unless they are actively invested in some sort of productive scheme, they are just sitting there (e.g. as huge estates, savings etc.).
In any case, it's much better for the economy to have a large middle class, than the equivalent money in fewer rich persons.