Fair point. Just because something has not happened does not necessarily mean it won't. At the same time, some things that have never happened will also never happen. But saying "I can not foresee an advancement in technology" is very different than "I can not foresee everyone agreeing with the same subjective values". Slavery is one such subjective value that was never, is still not and may never be universally agreed upon.
And yet, we've progressed to the point where but the most uneducated agree upon slavery being wrong. We've explored and will continue to explore and understand what is violence and abuse as we move on to understand what drives us. You can see, a harrowing example as it may be, male rape being talked about more openly nowadays; if yet unaccepted, it's a topic that people are willing to raise and discuss, not merely push away since "men can't be raped" as it used to sound before. That's merely an aspect of how we discover and develop as humanity. I prefer to think that it's going to progress to the point where subjective disagreements can be resolved through communication, even if it won't be in our lifetime. We have, after all, come a long way since the dawn of civilization - signified by the fact that people from different backgrounds like you and I can discuss subjective perception of such a vaguely-defined thing as tolerance and intolerance.