Assumes that the public benefits from whatever investments rich people make. Sometimes yes, sometimes no. So to give a few examples:

• Rich guy invests in factory that employs 1000 Americans. Obviously yes- clear public benefit.
• Rich guy invests in factory overseas that employs 1000 Chinese. Not so directly beneficial, though I suppose Americans get to buy some of the stuff in the factory. (Kind of depends on your broader attitudes towards trade).
• Rich guy invests in building condos. Housing supply increased, obviously yes.
• Rich guy buys an extra pied-a-terre (or five or ten). Here, there may actually be negative impacts, because rich guy is driving up the demand for housing.
• Rich guy invests in stocks for existing companies. Could be good because companies have more capital- on the other hand could he be creating an asset bubble?