I am employed and making more than 100K and still I am not doing anything productive for society. So even if I quit given UBI and become "unproductive" it will have no meaningful impact on anything.
You're presumably doing something productive for somebody who's willing to pay you $100K for it. Or do you work for the government?
It's complicated but in general if somebody is willing (or has to) to pay you it's not per se productive. In a society where everyone's basic needs are guaranteed to be covered we both would have made different choices.