To the slave owner yes, but their entire productivity is lower, it's better to have a businessmen rather than someone who can just pick cotton and do basic chores.
I think capitalism has a hand to play, to ignore it is naive. I think there was not one driver that ended slavery, but many things working together.
Probably not to the slave owners, or anyone involved in the slave trade. They had little incentive to change.
Claiming that capitalism somehow spurs these things is the wrong way around. It can certainly benefit, but it's not the driver.