"The point is, money spent on consumption is toxic — value-destroying" This is something I've been thinking about a lot recently. Surely when comparing inequality in societies, what we really care about is consumption. Singling out billionaires as an example of how unequal our society, I feel, is disingenuous. How much more does Bill Gates really consume than your 'average Joe'? Sure, he presumably eats good food, we…
The Gates foundation directs a large amount of resources not to "prudent economic decisions" (eg, it looses money), but to non economic priorities he set.
I have no particular complaints about the Gates foundation specifically, but in an idealized democracy, this scale of resource utilization would be subject to more democratic oversight (eg. Compare the CDC fighting malaria with appropriations from an elected congress to the Gates foundation doing so with appropriations from Bill Gates).
In practice, there is a lot of overhead to the democratic proceds, so we want more private charity then is ideal. And you are correct that there seems to be a limit on how much resources people consume before they start helping others (even if the way they help does not match the priorites of the general public)