I think its a bit harsh to call the papers "filler", but the reality is that most papers (in CS, anyway) are incremental work on important but well-studied problems or work on problems that are fairly narrow or not universally considered to be important. Reviewers tend to have wildly divergent opinions on how important or interesting that kind of work is.
The "in retrospect" was an important part of that point. Reviewers don't have access to it when reviewing.
Some conferences and journals have a retrospective prize for the best paper of, say, ten years ago. It's a neat way to recognize papers that turned out to be useful.
