The real fear is that if they can't threaten people with starvation, they can't get people to work.

Basic income shifts power from the employer to the employee. If you don't want Bob to quit his job, you need to give Bob an incentive to stay. You need to treat Bob better. (Yes, some people would quit anyway, to go do something else).

But then also there's the possible other Bob ending. Bob starts brewing his own beer, other people like Bob's beer, and you have a new microbrewery for people to enjoy...which is the other huge advantage of basic income.

It frees people to take creative risk.