Even if Gates gives his money to ostensibly non-political organizations, he's still deciding how an enormous amount of power is transferred.
Even Steve Jobs' widow, who is giving small amounts of hundreds of organizations, is reshaping society according to her own design. While I'm aligned with her values for the most part, it is still terrifying to think about all the Powell-Jobs' who have billions that they are spending on things that harm us all.
I don’t know that I love the idea of billionaires being the solution to those two problems, but I do think that some part of our resiliency as a nation is because the government doesn’t have total control over those things.
Not to mention the obvious conundrum that this may be great when the party you support is in power but not so great when they’re not.
If your assertion is that nobody should wield nation-scale power, then I agree, and welcome to anarchism!
If your assertion is that billionaires are an effective check on governmental power, I'm... not seeing it. Particularly given how much power billionaires have to influence the government itself.
For example, pharmaceutical companies in the US have killed millions of people through marketing and regulatory manipulation.
More to your point: as long as politicians are democratically elected, they can't brutalize and kill with impunity.
Now I need to go wash the fact that I just used "brand identity" in a sentence off my hands.
It's nice that Bill Gates uses his billions for tackling diseases, but he shouldn't be able to decide the fates of millions of people unilaterally. He's an individual wielding nation-scale power, and that's dangerous for everyone else.
The same is true for Musk, and it's especially true for the billionaires who are distorting our democracy (Thiel, Bankman-Fried, Bloomberg, Koch, Hastings, etc.)