If that transatlantic slave trade was ended out of the benevolence and good will of the slave masters, what would come next wouldn’t be colonization, but globalization where there’ll be: fair trade, cultural and social equality, racial redundancy, religious tolerance and national sovereignty.
One of the reasons it ended was because western societies had transitioned from manual to mechanized and industrialized societies at the heart of the Industrial Revolution which was why the need for humans resources transitioned to the need for natural resources; hence, the extractive colonial era.