Philanthropic foundations are certainly preferable to estates or family businesses, because of the implied external benefits to the public at large. However, wouldn't it be even better if the benefits were not just received by the public, but steered by the public as well? In fact, if we imagine public management of such funds, then we need only have a political process in order to establish a management policy. But, as you can see, that's just taxation! So we are not really so far off from an improved alignment of incentives.
Why are you interested in keeping billionaires rich? They are orders of magnitude richer than you, and you are to them as an ant, or perhaps a cute kitten, and yet you defend them. Are you really so insulted at the idea of sharing the wealth of our nation with its people?