The discussion is around sustainability: when people quit paid work they no longer contribute to the very tax base that is necessary to sustain a basic income.
Most UBI proposals I've heard of are the equivalent of $5.00-7.50 per hour wage. If what you imply were true - that upon achieving that level, people simply said "goal reached" and ceased to be further productive - then the USA median hourly wage would not be $22.
Even if UBI is set so low that it's not sufficient to live, change still happens at the margin. You will have people who were thinking about retirement that may now realize that with UBI they can bridge the gap until they get their full pension a few years from now. There will be people who are unmotivated and will now choose temporary work for part of the year and rely on UBI to make meets end. People work because they want money; reduce that incentive and fewer people will work.
That argument still relies upon the debatable premise that less formal employment means less human productivity.
For example, those "bridging the gap until their pension" are as likely to be reducing childcare costs (which are otherwise often subsidised by government/tax) for their descendants, spending more time on their own health, reducing the $health burden upon government, and any number of other potential reductions of the need for government spending. In equal proportion to the reduction in formal employment load upon the individual.
