I think that the work that needs to be done, will still get done but it'll be better rewarded (ie it'll be more expensive) - jobs that people don't want to do need to get paid more because employers can't just rely on economic coercion of the poor to force them to do dangerous or boring jobs, they need to remunerate them accordingly - which IMO is how the system should work. Result would probably be that businesses whose only value prop is predicated on cheap labor, will suffer, and businesses whose value prop is predicated on quality, will be rewarded.
EDIT: I do think there is risk that the coercion will become more intensely focused on illegal immigrants and foreign labor to fill in that niche, however, so better safeguards for illegal exploitation probably need to come along with UBI.