If the money stays in the bank it can at least belendt out and used for productive stuff. If the billionaire instead decided 'I will use 50 million to make all these people do something which is a complete waste of time' then he had used his money-given power to waste a bunch of people's limited time on earth.
In the end it's the money actually used in their life which is the actual manifestation of their power. Some of them might use it to do good, but if it's just decadence then those money do more good in the bank.
Firstly billionaires don't have "cash" in some bank account. (They are more likely to borrow from banks than to have deposits in banks.) Their assets are likely to be in property or stocks etc.
Secondly there isn't a "limited" amount of money for people to borrow. Its not physical. As a first approximation you can treat the lending ability of banks to be unlimited (at least for Joe's like me.)
For your first point, I agree thats how their assets often are. And it's better that they stay there than being used for dog-chef's.
For the latter I must admit that I don't really know how it works in the US. In my country banks have rules about the ratio between debt and capital, and if someone put in 100 million it would absolutely mean that the bank would be allowed to lend out more.
But in any case the point is pretty much the same! That the billionaires money do more good where they are, not when spent on frivolous shit.
There are million lot of people (all the way down the food chain) that benefit from what you call "frivolous".
Indeed there are whole industries that could be categorised as "frivolous" depending on your point of view.
Flowers for example. Loads of people are employed growing, transporting, storing, selling, arranging them, yet nevertheless we have them, and people buy them.
Is me buying flowers any different to a billionaire buying a yacht?
Think about money as voting power for what the collective production force does.
You buying flowers is a tiny bit of vote for someone making flowers, giving you happiness. Someone buying a dog chef is a significant vote (enough for a whole worker) for... two happy dogs?
I don't think about employment as a 'gift' from the ones employing. The dog chef could, and would, have done something else if they did not cater to a couple of dogs. The billionaire gets to make another human waste their production because they have such immense voting power with all their money. And yes, the dog-chef might be happy, or maybe they are just a wage-slave doing what they can to survive.
In the end it's the money actually used in their life which is the actual manifestation of their power. Some of them might use it to do good, but if it's just decadence then those money do more good in the bank.