You miss interpret my comments.
I'm stating that using manual labor is the MOST expensive means of producing a crop - any crop. Platation owners were business men, greedy men. Argueably some of the most greedy ever as they earned their profits off the work and efforts of others - without compensating them for those efforts.
However, if there were a less expensive means of producing the crops, there is no doubt in my mind that plantations owners would have moved to reduce their slave holdings. Simply because it would have been expensive to maintain slaves.
Understand they would not have done so out of any charity, simple economics would have ruled the day. Given a choice between more expensive manual labor or less expensive machinery, farmers always have and always will choose the less expensive option.