You are saying that billionaires do more than consume; they save money. If we wish to be more specific, we could say that billionaires, by and large, invest money, billions of it, in enterprises which will make them more money.
Economies don't grow from some abstract demand existing, they grow from production (of final goods and services). Investment in productive assets is the key to the economy's growth.
Obviously some billionaires do more to be leeches than others, but that sort of enterprise seldom needs actual net investment. And obviously not all capital comes from billionaires, and not all billionaires deserve as much money as they have — but your premise here is quite backwards. A billionaire consuming all his wealth by building mansions, showy superyachts, private planes, and the like, is much worse for the economy than a typical billionaire investment, because the superyacht is not a productive asset, it is an extravagant waste.