For reference, Bill and Melinda Gates have not been able to eradicate polio, despite pouring billions into defeating it (though perhaps not 10s of billions yet, into that specific cause).

So worth considering --  how do worldwide climate change and US homelessness compare to that?

You can't do anything meaningful on the country-wide scale for $20bln. The US social security budget is over a trillion dollars, the highest budget expense, and it is not nearly enough. Your "progressive" (read ideologically left) friends are posting it for the feel-good ingroup applause lights without ever checking the sources. Not very much different from "covid is a Chinese/Liberal conspiracy".

I don't have sufficient knowledge to debate specific numbers, so I will just point out the simple things.

First, where are the governments in this picture? Governments operate with billions and trillions, so by the same logic governments also don't care about the problems mentioned. And by the way, governments already take money from us under the pretense that they are going to use it to solve exactly this kind of problems... so, if those trillions collected each year haven't solved the problem of health care or homelessness yet, what exactly makes you believe that the extra 20 billions would?

Second, the picture describes what could be done, but fails to mention what has already been done and how much did it cost. Maybe things are actually way more expensive than the picture suggests. (Like, the picture represents a planning fallacy, and in reality more money was already spent and the problem is not fully solved yet.) Maybe the solved problems keep re-appearing. (Like, if you gave a new home to every homeless person, a year later, most of them would be homeless again, for various reasons.) Maybe simple solutions have bad side effects. (Donate money to lots of people... and watch the rents go up.)

I am not pretending that this is a full refutation of the argument. As I said, I don't really have sufficient knowledge to do that. I am just saying the evidence is filtered so much, that any conclusions made from it are unreliable.

Bill gates probably thinks that malaria in Africa is a better opportunity to do good than American homelessness. Elon musk chose electric cars and to give some money to openAI. I think the pattern here is that some billionaires do use a significant fraction of their fortune to do good, and they are fairly effective at doing so. But you can still find lots of important causes that aren't being funded by them, because there aren't that many billionaires.

(I've never understood this line of reasoning. But again, a lot of people here take that argument seriously, which probably means I'm making some mistake here. I had never been able to get what it could be, so if someone can explain where is the issue with my reasoning, I'll be glad.)

tl;dr We should taboo "money redistribution" and instead only talk about "consumption redistribution" and "power redistribution", which are two almost-separate things. I argue that in light of none of them, it's a good idea to take billionares' wealth.

Ok, so, what matters is consumption. A billionare does not buy a million cars, or sausages for BBQ, and generally there's not much difference in consumption between Warren Buffett and "just" very rich Beyoncé. [citation needed]. Redistribution of wealth, even if it was very liquid, does not cause additional economic output to appear. It might happen in a short term, when everyone is getting this $10,000 and spending it, boosting the economy, but it's the same effect as increasing money supply by a central bank. In the long term, any effect that can be caused by such intervention (of wealth transfer) can also be caused by Fed policy. The only way we could really make a change would be to set an extremely high tax on consumption. And most of the consumption that we would redistribute is done by the middle class.

If the billionares money does not go to consuption, where does it go? It's invested. So maybe the reason to redistribute is not to generate output, but to limit the power of the rich. But the free market is specifically set up in this way to transfer the power to manage money from the incompetent to competent - and that would be exactly reversing this trend! If Joe The Plumber gets a voting share in some company, he will most probably make worse decisions than Warren Buffett. Personal freedom is of course dependent on economic freedom, but it's a far-reaching link - if Joe controlled a voting share in the company he personally works, he could make a decision to switch from plumbing to gardening, or laying and watching TV, which would free him from the power Warren Buffett currently has over him, forcing him to do plumbing instead, but this is not because Warren is - it is because it's an optimal market decision, and he, coincidentally, is just an executor of this will (again, because the market granted him this power through Darwinian process of wealth redistribution to people most competent in generating/holding into it).