The slaves' comments in the WPA interviews don't particularly surprise me as I've heard similar sentiments from other people who, by any objective analysis, would seem to be living in better times than once they did, but still look back wistfully on those earlier times. For example, within a few years of the Soviet Union collapsing you could find many older people professing that they wished they were back in the old Soviet Union. I don't think it's that they've forgotten the details of their former lives but that somehow time softens the harshness of the day to day specifics; and this, combined with the sad fact that most of us become less adaptable to change as we get older, can make earlier times seem perversely attractive.
I think Hilary Mantel summed things up rather well in an essay in the Guardian a couple of years back when she wrote, "History offers us vicarious experience. It allows the youngest student to possess the ground equally with his elders; without a knowledge of history to give him a context for present events, he is at the mercy of every social misdiagnosis handed to him. The old always think the world is getting worse; it is for the young, equipped with historical facts, to point out that, compared with 1509, or even 1939, life in 2009 is sweet as honey. Immersion in history doesn't make you backward-looking; it makes you want to run like hell towards the future." http://www.guardian.co.uk/books/2009/oct/17/hilary-mantel-author-booker