The phrase "won't fly" is often used to express skepticism or rejection of an idea, plan or proposal. Antonyms for this phrase could include "will succeed," "will take off," "will work," or "will be accepted." These antonyms suggest that an idea or proposal is likely to be successful or well-received by others. Other antonyms could include "valid," "prevalent," "proven," or "acceptable," indicating that an idea or proposal has already been accepted or established as a viable option. Finally, antonyms such as "grounded," "unrealistic," or "unfeasible" suggest that an idea or proposal is unlikely to succeed or gain traction.