Many currently receiving benefits might gladly give them up in exchange for $1k/mo, cash in hand, with no risk of losing benefits due to higher income or filling out a form incorrectly.
But even if not, I think there's a case that giving everyone else UBI has ancillary benefits:
- More cash in the hand of your neighbors means more customers, which means more economic activity, which potentially means better employment prospects, in a virtuous cycle [0].
- Strain on existing benefit-granting institutions is greatly reduced, as those who opt for cash exit the system, meaning faster response times and more assistance with forms/approvals/etc.
- The working class tend to be more economically interdependent by necessity; someone who declines UBI still comes ahead from a spouse, relative, grown child, etc. who requires less economic support.
- Yang has spoken at length about the effects of economic anxiety; even someone who declines UBI may experience less anxiety (and therefore greater executive function and capacity for long-term planning), simply from knowing they have a "Plan B" for basic necessities.
In addition to offsetting costs, I think one of the motivations for the "either/or" strategy is political viability with libertarians and moderate Republicans, whose exaggerated fears of "socialism" can be assuaged by the opportunity to shrink bloated federal bureaucracies. (I'm somewhat sympathetic here: the most efficient charity is usually to write a check to the poor [1], and I suspect UBI or negative income tax has greater efficacy than most means-tested federal programs, with possibly the exception of health care.)
[0] https://en.wikipedia.org/wiki/Demand-side_economics
[1] https://www.givewell.org/charities/give-directly