That really just furthers the point that we need UBI in my mind.

The people who are making today more or less the same as what the UBI would be would have their income doubled overnight. And yeah, some will say fuck it and quit their jobs to just lounge around (though I imagine many will go back, ask anyone whose been out of work for a long time, it gets boring quicker than you might think), but I'd wager most will take that double income and run with it. Twice your takehome would be life changing for just about everybody. Hell, those who continue to work will probably wind up with more than double, because demand for those jobs will go up.

Jobs that are unpleasant or difficult will basically start actually getting paid what they're worth, because no one will be stuck in a "I have to do this or starve" situation.

And yes, the overall GDP probably will take a hit, because we won't be working our population to death, but productivity has skyrocketed over the last century, it's about time we start putting that fact to work for the actual people, instead of using it to extract record profits for the top 1%.

TL;DR - People will still work because working will still mean more money. Some won't, but that's fine. If jobs are having a hard time being filled, then employers will simply have to pay more to get them done, or explore ways to automate the parts people don't want to do