The book The Better Angels of Our Nature by Steven Pinker documents the decline of violence over human history, and in lectures about it he's noted that many liberal/progressive people seem reluctant to accept this idea out of a fear that it could lead to complacency. The thinking is that if people don't think the world is awful they won't be motivated to improve it. And I'm pretty sure he points out the converse can also be a problem, if people think the world is awful they can become hopeless and apathetic.
Which is why I think perspective is so important. On the one hand we've come a very long way and on the other, we still have a very long way to go. It should be trivial that we've come a long way, just a century ago women couldn't vote, just a few decades before that people owned other people as property, right here, in America. The word genocide wasn't coined until 1943/44. While it's true there are still a lot of people who would oppose any of these improvements the fact that they're unthinkable to a large fraction of the world today is what is important. Just a few centuries ago they were the norm, and there were just a rare few people who could even imagine a world where these improvements were made.
But that isn't to say there are no more problems to solve, or that today's problems aren't grave injustices or aren't worth our efforts or attention to improve them. In America, minority rights have always suffered. Mass incarceration, racial profiling, police shooting unarmed black men without consequence, mandatory sentencing, redlining—the list is long.
Which is why I think the phrasing, “we've come a very long way, but still have a very long way to go” is so important. The first half is important for reminding us that moral progress is possible, the world can be made better, we can change and improve it, and we should not be hopeless or apathetic. The second half is important in reminding us we cannot be complacent, there are still grave injustices that demand our attention. It's also important to understand and remember that while moral progress is possible it is not inevitable, and I believe Pinker has been clear about that in the lectures he's given.