That's provably false.
https://en.wikipedia.org/wiki/Principle_of_least_effort
https://medium.com/the-mission/dont-use-rewards-for-motivati...
http://hepg.org/hel-home/issues/10_2/helarticle/the-case-aga...
"Rewards kill creativity. Some twenty studies have shown that people do inferior work when they are expecting to get a reward for doing it, as compared with people doing the same task without any expectation of a reward. That effect is most pronounced when creativity is involved in the task."
Would this not suggest that people do their best work when they aren't doing it for the paycheck? In other words, given UBI, anything an individual may do would be his or her own prerogative and therefore represent higher quality output.
But having fun making art means little to an economy.
Having less fun designing a robot to pick potatoes will transform an economy.
Creativity is nice, but it doesn't directly correlate with additional value.
The proposition was that people will do the least work for the most reward where possible.
The cited article suggested that rewards result in poor quality work.
(EDIT: This is a genuine question; I'm not highly educated on economics and I'd like to know the approximate models that currently exist on how much growing the economy helps numbers like homeless population, healthcare coverage, lifespan, birth survival, food security...)