Like I said, financially I can agree. However, from a quality and enjoyment of life standpoint, I couldn't say.
Unless we are saying that no one from the African countries that our slaves came from has a happy life and that no one descended from slaves in this country has a bad life, it's not an easy thing to quantify. If you are using happiness as a quantifier, there were happy slaves. Doesn't mean they were well off but, if we trust their own words, they were happy.
In general the people in the US may have better lives than those countries, I'm not trying to say otherwise. It's when one basically says that all people in the US have better lives than all people in those African countries I take issue.