A flat tax combined with a basic income would be extremely simple to administrate. It would save money and shrink government. If we instead decided to only tax people if they earned enough, that would require calculating everyone's income which would require government workers, and it would also require targeted welfare which punishes people for working via withdrawal of benefits in reward for working.
As for thinking everyone will do nothing, I don't know what you're talking about. People sell their labor in the labor market because they have no other choice, unless they are born rich that is, they have a choice. Only one third of the labor market in the US is engaged by their work. Do you think that is evidence people love what they do?
I don't at all think people are lazy, nor do I think people should be forced to work involuntarily. I think work should be fully voluntary, and that the work people do is what they choose to do, for enough money to get them to voluntarily do it, or for no money because their basic living expenses are already met.
Why have an income tax at all? It should not be mine or the governments business as to how much you earn. Yes, even rich people work. Try not to justify programs with jealousy. People have to work to eat, nothing changes that, not even printing money out of thin air. Without products being first produced, they cannot be consumed, no matter how much Zimbabwe money you have.