The claim a UBI erodes personal incentives for self-improvement and hard work ignores the psychological and/or sociological components of these traits. It assumes such desires are externally motivated. These traits are likely a mix of internal and external factors. For those highly internally motivated, UBI is less likely to substantially change their behavior.
There are other incentives for self-improvement, such as a sense of accomplishment or mastery.
The thesis presupposes that human beings are primarily motivated by their own consumption - that they only do anything if it increases their ability to consume. But humans are primarily motivated by an existential calling. After all, one would not be interested in sustaining their life if it did not have meaning for them.
In the last years the concept of gamification emerged. It shows that motivation often has nothing to do with money, but rather people can develop a desired behavior through an activity that is meaningful. This can reward people without necessarily using money to do so.