Well to be clear, I don't think this is in any way a great solution. The idea of just giving people money that they haven't earned presents a shit ton of problems.
But I think the ultra rich have a problem that some of them have recognized more than others. Money is essentially meaningless to them now because they literally can't stop making crazy amounts of it every day. They're making money in an hour that could support entire lineages.
So eventually they have to focus on societal stability, and when you have a ton of people who cant afford to live, it kinda makes sense to just give them enough money to live.
Ah, so more of a welfare-type of basic income, not a true universal basic income (UBI) for everyone on Earth.
Forgot to mention earlier (and I'm going off on a tangent here): a big difference between utopian far-future post-scarcity UBI and near-future "tax-the-rich" UBI is that post-scarcity UBI assumes human labor has been largely obviated by AI automation, meaning the vast majority of humans don't need to work, and there's not that much work available anyway. Most "jobs" for humans would be voluntary, for fun or personal development or just having something to do besides jacking off all day. This is basically Star Trek society. Most things anyone would need are readily available through replicators, AI does most of the menial work that keeps the world running, most doctors and surgeons and other white-collars have been replaced by AI, etc.
Meanwhile, in the current-day "tax-the-rich" version of UBI, the world is obviously only partially automated, and humans are still needed for most jobs that keep society running. So what happens when large swaths of the population decide to live on UBI alone, and quit those jobs? Suddenly, getting an oil change costs $400, and has to be booked a month in advance, because there are only 1/5th the previous number of greasemonkeys working out there. Maybe that's a naive view; I haven't thought much about this or read anything on it.