You don’t seem to understand the history you’re referencing. Slavery and mass displacement / ethnic cleansing aren’t mutually exclusive, they are mutually interdependent. Empires engaging in settler-colonialism didn’t choose one or the other, they did both, always. Even if they outlawed slavery domestically, they still participated in the trade internationally or in their colonies. Settler-colonial empires still engage in slavery to this day, they’re just better at hiding and justifying it. See: the US prison system and abuse of migrant workers, and the kafala system in the middle east (called the “binding system” in Israel until it was de jure abolished in 2006, but de facto continues to this day in a sort of legal gray area). These days the word slavery makes people squeamish, so we call it things like human trafficking, prison labor, migrant labor, and all sorts of other more polite euphemisms to lull us into the false notion that slavery is a thing of the past - or at the least relegated to a tiny secretive black market.