My friends, generally speaking, spent a lot of money on a very good education that's not valued by the labor market. To put that another way: My friends are wildly overqualified for what they do, and many of them are poorer than broke.
Those without a lot of ambition are pretty much the millenial layabouts you imagine. They're working median-wage retail jobs to pay the rent, smoking a lot of weed, and just generally hanging out. They don't want to work more, and they couldn't really work any less, but they seem pretty happy.
Those with ambition aren't living much different. They're working median-wage retail jobs to pay the rent, working second jobs to try to pay down their debt faster, smoking a lot less weed, and using the rest of their time trying hard to find a job in their field of expertise that wouldn't pay much more even if it did exist. These people could easily work a lot less if they wanted to, but they don't. They want to work more, and work harder, but they cannot find work to do. They seem like they're struggling.
So say we gave them all an unconditional grant which erases their debt and provides some capital. (A year's income wouldn't do this for most, but set that aside.) Most of the first group, maybe it wouldn't affect that much. They might quit their jobs or cut down on hours, but actually they don't mind their jobs that much. They might smoke more weed, but that's probably not possible. More likely they'll spring for a car or a house or a home theater and just keep on keepin' on.
For those in the second group, though, this changes everything. They've instantly jumped a decade into their own future. They'll quit their jobs the same day, immediately start planning a move to where they really want to live. They'll immediately open small businesses. They'll collaborate on epic works of art. Some of them will buy boats; some of them will buy farms. They'll travel, volunteer, teach, research, write, direct, design, produce, and make things. And you know what? They'll probably smoke even less weed.
It's just a wishful thought experiment, but it does seem plausible the overall economic effect would be massively positive. The argument against basic income seems to be basically that it would move people from the second category into the first category. Maybe that's the case for people in general, I don't know, but for the poor young people I know it seems far more likely to do the opposite.