Billionaires are mortal.
Money isn't.
When Jeff Bezos dies, someone is going to inherit his money. There's no particular reason to believe that whoever he chooses as his heir(s) will be less disposed toward funding fascism and destroying the ability of regular people to have a voice in the world.
You invoke Carnegie, but the current generation of robber barons have shown no real propensity toward philanthropy—they donate to charity, to be sure, but in tiny (relative) amounts, and frequently to their own foundations, which make it legally charity, but in practice just another way for them to control their own money.
The proposals that the billionaires of today will give away their money (and thus power) within their lifetimes, or that their heirs will suddenly become champions of labor and the downtrodden, require some fairly significant evidence—from today, not just comparisons to the Gilded Age—before they can be considered credible.