Scenario: You're married with 2 kids and live in San Francisco. Is an $8,000/year UBI going to enable you to quit your $100,000/year job without another lined up?
The more you make the more you can save. And the more you can own. People who make that much aren't the ones you need to target to improve their bargaining power or labor mobility, but UBI is still a benefit. Some cash flow is better than no cash flow. Even if it's only enough to pay the interest on their credit cards, that's still a big deal. And might be enough to convince people to leave jobs they hate.