There is a general problem in mass psychology where people sitting around a table or in their armchairs try to imagine the impact of a policy without conducting a study or looking at historical results.
Let me present some more historical results: retirees. Do pensioners contribute more or less to society than before they retired? Are they a net contributor or a net drag? A UBI turns everybody into a pensioner.
The two situations are not identical, but they give me pause.