That's the assumption that most people make when considering UBI and even though I don't fully agree with it, the fact is that a percentage of people will do just that; sit on their asses and wait for their paychecks to arrive.

When no financial incentive is given to work, what makes you think that people will willingly go out and work for "nothing"? Because in the end, they will all get the same fixed amount whether they work or not. If that amount is enough for them, they will retire and chill for the rest of their lives.

All systems can be gamed. We would have to think really hard how to prevent and minimize it before implementing any result-based reward distribution system.