Huh? Nobody's suggesting a 100% rate. Every dollar you earn increases your income with a negative income tax, it just doesn't increase it by a full dollar. It might be 90 cents. Still a fairly strong incentive to work.
Isn't that directly contradicting the point you made earlier? If every cent counts at the lower level, then even earning 90 cents on the dollar extra is still plenty incentive to attempt to earn more.