Re: It is impossible to have a billion dollars without that fact in some way helping others.
It's a massive misunderstanding to think these trillions could ever exist in actual liquid cash. It's all estimates of the value of assets, and that value is only as much as anyone is prepared to pay, which is in turn reliant on how much is getting sold.
If the "400 upper-crustaceans" attempted to sell their assets, in order to give anyone on the planet three hundred bucks, who would they sell it to? Who has $2.02 trillion to spare? No-one. But that's not a concern, because as soon as they started selling their assets, the value of them would plummet. The market becomes flooded, everyone is wondering why you are selling, economies crash, the actual trillions of dollars become worth far less than they were the day before, everyone loses in the resulting chaos.
The fact is these guys are only "worth" $2.02 trillion as long as they never, ever, try to realise their worth. Which is not to say they aren't very rich indeed in a material sense. But the actual figures bandied about are little better than guesses and estimates of a scenario that could never happen.
Apart from that; if you give everyone in the world $300, inflation immediately makes everything cost $300 more. You only make people richer by creating assets (products, services, skills), not by converting them into cash.