Very interesting result, and it makes you feel really sorry that most of those old papers are behind paywalls. (Incidentally, this is a problem which is not goi

Very interesting result, and it makes you feel really sorry that most of those old papers are behind paywalls. (Incidentally, this is a problem which is not going to get solved even if everyone switched to open-access venues today.) Just a comment though: not all citations are equal, so just counting them is quite a crude metric. For instance, a lot of citations in my field (theoretical CS) are "attribution citations" that point the reader to the original paper that introduced a concept or proved a result; and these are not the same as citations of work that you actually extend, or to which you compare. As in theoretical CS things progress fast and people usually improve upon fairly recent works, my feeling (not backed up by data) is that most citations of work older than 10 years are attribution citations; and for attribution, you don't really need to have read the original paper, you just need to know what it introduced or proved. So maybe the Web is making it easier to look up older papers and cite them, but it doesn't mean that the older paper will influence your research beyond adding a bibliographical entry. You could say those additional cites may be useful to the reader, but even then, readers unfamiliar with a concept would often do better to find a recent survey about the concept, rather than try to understand the original paper that introduced it. (The original paper is usually hard to read because it is old and language and notation have changed; and people probably didn't have a good understanding of the concept when they introduced it.) So those citations are mostly for courtesy.