That is the problem with link-based PR. Years ago when Google and other SEs figures out that links provided better results than word frequency or other methods of determining their SERPS, it worked fine, but like you point out - after a while the top SERPS tend to be dated.
A few years back Google started popping new articles to the top (Freshness). It gave the new pages a chance. But if some professor linked to a site several years ago because at that time it answered his question (giving the site an .edu or gov inbound link), then it will likely still rank towards the top.
Hopefully it is one of my kids that figures out a better way to sort SERPS. LOL