I disagree. I think we do have ways to think about what future generations will judge us harshly about.
Obviously some things we can't tell, like what clothes fashions will be in 50 or 100 years. But when you get down to matters of morality and ethics, I believe we do see a clear progression over time. And we can predict it, to some extent. That's the first reason I think we'll be judged harshly here.
A second reason is that we can already see this trend happening. More and more people care about cruelty-free farming, as well as are becoming vegetarians or vegans. Of course, that trend might stop or reverse. But we have reasonable grounds to think it might continue. If it does, then eventually current society will look pretty bad.
A third reason is that this isn't an arbitrary issue that we happen to see a trend on. A lot of people that eat meat realize there is something wrong with things like factory farming. They ignore it, and it's easy to ignore for the most part. But when they think about it, they realize it's immoral. Most people are good in the sense that they do not want to harm other living beings; caring about animal suffering is just a natural outcome of that. In other words, most people don't need to be convinced on the merits of the issue. What most don't yet realize is the huge extent of the problem, and that there are good alternatives.
See also this article for further reasoning along these lines: http://www.bbc.com/news/magazine-35667355