If Huang Gai had failed or not been sent, Zhou Yu and Zhuge Liang would have just cooked up another scheme. Cao Cao's troops were rotting from disease and other debilitations by this point and could not last in a pitched battle on the river. Time was on Wu's side, so even had Cao Cao seen through the scheme, Huang Gai and Kan Ze would have been executed, but Wu still would have found a way to prevail, given Cao Cao's problems in his army.
"To the meaningless French idealisms of Liberty, Equality, and Fraternity, we oppose the German realisms of Infantry, Cavalry, and Artillery."
- Prince Bernhard von Bulow, German Chancellor 1900-09 (attrib.)