It was not the manpower cost that was the limit. Steam ships had huge crews managing the engines. Titanic had 25 engineers, 170 firemen, 73 coal trimmers and 33 greasers.
It was the size. Sail ships had practical upper limit for their size and weight. Size of the mast and sails could not just grow.
Square–cube law: (drag) surface area is proportional to the square while the volume is proportional to the cube. Bigger the ship, more efficient it becomes.
It’s important to realize that 300 man engine crew was needed in large part do to the Titanic’s fast 21 knots cruse. A slower vessel of the same size and weight needs dramatically less power. By comparison the last commercial bulk cargo sailing ship Palmer which sank in 1957 needed an 85+ man crew, only had an 8-9 knots cruse, and was vastly smaller. For the titanic to add an extra 200+ crew to possibly save less than 10% on fuel costs was simply not tenable.
Where combined steam and sail ships really benefited wasn’t fuel, it was consistent power. Early steam engines where unreliable as was sail, but they rarely failed at the same time. However with more reliable steam it was just a question of economics.
There's a Liberty Ship that sails out of Baltimore at times [1] and if you go down into the engine room you can meet (or could 3 years ago) a greaser who sailed with these ships during WW II. They were the last mass-produced steam ships in the world [2].
It was the size. Sail ships had practical upper limit for their size and weight. Size of the mast and sails could not just grow.
Square–cube law: (drag) surface area is proportional to the square while the volume is proportional to the cube. Bigger the ship, more efficient it becomes.