I may be misguided on this, but I don't believe a critique of UBI is the reduction in an individuals willingness to work, but rather the act of working. Individuals who don't want to work but do so as an understanding of a means to provide for themselves still have an unwillingness to work that wouldn't change if provided UBI, but their likelihood of quitting their job and not filling that time with a productive replacement is what is being questioned by opponents.