The literal purpose of UBI is to ensure that taxpayers do owe each citizen a fixed amount set at an average living.
Not average. It's Basic. It's just enough to get by. That's the incentive to work: most people want to do more than just get by. Anyone who doesn't want that would be a drag on any system, so that's a separate problem to deal with. But there are a lot of people who can't get ahead with the current system who would do better in a UBI system, and we'd all be better off if we could get those people productive and happy.