I'm more concerned about people in governments, not rich millionaires. The only way it could work would be to create a UBI of 0 at the beginning and publicly say to increase it only every time a social welfare is deleted. I have huge doubts about the doability of this solution.
I don't have any issue with billionaires believing into UBI and sending cash to Kenyans. I don't know their true agenda, but they might gain some political influence / power when they'll try to implement this into western countries (and it will be with our money this time of course, not theirs).
I think you're working from some wrong assumption who these UBI proponents are, and what their motivation is. Just to give you an idea, Wikipedia lists Elon Musk, Tim Berners-Lee, Mark Zuckerberg, Sam Altman, Bill Gross, and Tim Draper, among others. These aren't lazy know-nothings trying to get you to finance their lazy Tuesdays.