That's the usual theory about people being inherently lazy and not working if they can avoid it. But that's not what studies say. The vast majority of people find work valorizing. Even if people might initially think this UBI allows them to quit work, it won't last for most people - they'll want to have work to feel part of society. UBI anyway isn't a free goldmine; it's supposed to be a liveable wage (possibly adjusted by location), which is really pretty minimal in terms of what you can get out of it.
Other than that, it does give people flexibility - for the better. A much bigger test done in Manitoba decades ago showed that people would more often go for additional education or training (bettering their professional position); might be less likely to work overtime and to do volunteering and make for family and hobbies instead (better for society and mental health).
It would also provide a better safety net. The US is often touted (certainly by people from the US) as the most entrepeneurial country in the world, but it's not; countries like Sweden do much better in that sense - because its social safety net allows people to take business risks without the same chance of completely ruining yourself financially for the rest of your life. UBI would provide a similar situation.
I know this wasn't your argument, but the inflation point (often brought up) is ridiculous. Enabling the unemployed and working poor to have a liveable wage does not drive up inflation unsustainably. It's a gross exaggeration of these people's economic influence. In fact, even current arguments that wages shouldnt match current inflation levels as that creates a vicious circle are nonsense: in Quebec, raising salaries by 6.5% would only add 0.8% to inflation. (
Link, in French.)
Finally, as for costs, there are two things to consider. First, as many pointed out, a huge government machinery could be dismantled if UBI were implemented, plus all other welfare payments, which would compensate for a lot of the costs of a UBI program. (Plus there's the advantage removing the social discomfort of having to prove you're poor enough to receive various benefits!) And second, for whatever cost is left - why shouldn't it be a government priority to ensure people have a liveable wage? If that means increasing the top tax bracket of business profit taxes, would that really hurt society? What's the priority here?