I don't know what you mean by "work ethic" (why does that have anything to do with UBI?), but regarding UBI, I think you're either exaggerating or misunderstanding some things, and I say these without any regard to my position on the topic (which is complicated).
When you think UBI, don't think "everyone gets $300k/year for free so nobody needs to worry about working anymore". Think more like "everyone gets $30k/year for free so nobody's life is paycheck-to-paycheck". There are lots of people who literally don't have the opportunity to do anything (even better jobs!) other than their current jobs, which they need to put food on the table and have a roof over their heads. One of the envisioned benefits of UBI is that it would actually give them some opportunity & breathing room in their schedules to do something else - whether it's raising a kid, learning new skills, applying for other jobs, or anything else. Most people would still have quite a bit of an incentive to work for a more comfortable standard of living - it's just that they won't feel like they have a gun to their heads as the alternative.
Does this mean UBI is obviously practical and we should start it tomorrow? No, there are lots of challenges, most notably including the funding itself - not many economies today are able to hand out that much money in a sustainable way, so there's some very hard rethinking necessary to figure out how this could ever work. Does this mean everybody will give a 10x ROI on it in the long term? No, some people would probably not do anything useful with the money. Does this mean UBI solves all problems? No, even if you do your best you can still have bad luck (get hit by a bus, medical bills, etc.). Does this mean UBI would work well everywhere? No, results might vary across different societies.
But we're rapidly heading into a world where more and more people are being put out of work, and can't pick up new skills fast enough (if at all) to make ends meet. Expecting everyone to just adapt is incredibly unrealistic. UBI is one envisioned plausible way to address this problem and others systematically. Nobody knows for sure how it would play out in the long term, but to my knowledge nobody has obviously better solutions either.