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.
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.