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.
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.