Many people may agree to work for less than the current minimum wage, and on more flexible terms, if they're supplementing a guaranteed income, not scrambling to avoid having to beg for food.
But this is not the case. What everyone seems to forget that despite the low labor force participation rate, the poorest are not starving to death; in fact, they're more likely to be obese than higher earners.
The assumption that a UBI, on top of existing welfare programs, will make people more compelled to work seems like a stretch.