Why would that be hard for people to accept? What is true is that whether they are or not can never justify people being made slaves. That is the problem with this argument, and by linking the "better now" with "enslaved" you make that argument.
I'm not justifying slavery. I am just saying out of evil happening to some people, many times other people live better lives.
The bad things that happened to many people in Europe caused some of their countrymen to leave and settle elsewhere. Many times they left devastated lands for America. Basically, those are the people who created America. If their lives were great in the old country, they would have never left. But they did, and they all helped create America.
Why is that difficult to understand?
Over the millennia just about every human development on earth was invaded/conquered by somebody else. It was obviously bad for the people conquered/killed/enslaved.
But the mixture of the conquerors with the conquered created whole societies. Many times those new societies created better lives for the descendants of the conquered/killed/enslaved.