It's interesting that slavery existed for millennia, and then virtually disappeared within a century or so of the emergence of capitalism.

I wouldn't say that, slavery is still existent as alternative forms around the world, not discounting the fact of so-called sweatshop slavery

So only committing an evil for a couple centuries means it's OK? As I still see it as a fine example of capitalism given the right to do something that was evil.

Capitalism does not necessarily imply in slavery, actually some could even argue the opposite: that a desperate mass of 'free' people feeds and operates the economical machinery in a much better way. That was the breakthrough of the Ford model (workers that could buy cars).

Of course, assuming that you meant the classical meaning of the word slavery.

This does not invalidate your general argument though.

Slavery existed long before capitalism.