Assuming you implemented UBI, and changed nothing else, what would prevent the market from adjusting overall price levels (upwards) until those living only on an UBI again end up below the (now increased) poverty line?
In other words, I believe that without penalizing the upper end, too, things will just "adapt" to define a new lower end, if only the only change is to give everybody a little bit of money. To some extent, Germany has been doing something close to UBI for quite a while now ("Grundsicherung", "Harz IV", etc.). It certainly helped prevent that a few people fall into some nasty global minimum with zero income, so its probably better than not having it. But I don't think that only clamping the lower income end alone had a society-transforming impact in Germany.
In other words, I believe that without penalizing the upper end, too, things will just "adapt" to define a new lower end, if only the only change is to give everybody a little bit of money. To some extent, Germany has been doing something close to UBI for quite a while now ("Grundsicherung", "Harz IV", etc.). It certainly helped prevent that a few people fall into some nasty global minimum with zero income, so its probably better than not having it. But I don't think that only clamping the lower income end alone had a society-transforming impact in Germany.