The vanishingly small amount of people that will be unfathomably rich in a privatized post-scarcity economy will give us just enough in UBI to make sure we can buy our Mountain Dew verification cans. And without the ability to withhold our labor as a class, we'll have no peaceful avenue to improve our conditions.
Except it's not. That wealth isn't cash in some bank account, in most cases it's a stock in companies these people built from scratch - Bezos made Amazon, Gates Microsoft, Buffet Berkshire Hathaway and so on
The wealth of super rich is allocated in places that produce goods and services