Literature tends to have a sort of timeless quality to it, in that you have "the text" and from there you can spin off to literary criticism, textual criticism, etc. In pulp studies, this means pieces written on stories or on the genres themselves can have a very long lifespan - The Simple Art of Murder still stands fairly well almost seventy years on. For pieces that focus on text, the pieces are still relevant so long as the text is relevant - that is, until a different edition of the text changes the reading. So for example, it was long accepted that H. P. Lovecraft wrote "ibn Schacabao" in "The Festival," based on the published versions of that story - but a later comparison with Lovecraft's manuscript revealed he wrote ibn Schacabac - a very small difference, but this more directly ties the character into a character from the "Tale of Bromecide's Feast" in the Thousand and One Nights. So that kind of discovery of a variant text can upset any text-based reading of Lovecraft that depended on that.

In much the same way, history is generally at the mercy of new discoveries - be they archaeological, or the uncovering of new letters or diaries that give different insight on a subject - and as the new data accumulates the older articles and essays tend to be more and more superseded. In pulp studies, this is generally true as more biographies and collections of letters are published; original research and easier access to source documents can make books and essays written before that new material was published/made available dated as well.

Then there is a...shifting of popular critical opinion. For example, one of the major biographies of H. P. Lovecraft was H. P. Lovecraft: A Life written by L. Sprague de Camp and published in 1975. Nobody would fault de Camp for his research (well, mostly) given the resources available at that point, but the major criticism was his approach in writing the book - his harsh criticism and appraisal of Lovecraft's life, person, and work, his efforts to posthumously psychoanalyze Lovecraft, etc. The tone and approach are a function of temporal syntax; historians are not immune to falling into the style and approach determined by their period (and/or field, since some fields have clearly-defined camps or schools around certain issues, or preferences regarding how certain subjects are approach - consider Saltwater vs. Freshwater Economics). So in the case of that book, it's not so much that de Camp is always factually wrong (though that is the case in parts), but that his method of addressing the subject fell out of favor (and largely continues to be out of favor), representing a changing trend in Lovecraft scholarship toward a more objective view of Lovecraft as a person (some might argue it's gone the other way into fawning adoration, but I don't think that is an accurate appraisal).

So, there's absolutely material from 30, 50, or 70 years ago that is still relevant today - and there is material that has been largely or almost completely superseded.

There's also the "meta" angle: the history of the writing of the subject becomes something of a subject itself; to properly judge and understand a piece written in the 1930s, for example, I need to be familiar with the prevailing materials and attitudes that the writer at that time knew, or would know. And anybody writing in the 50s would probably be referencing those articles from the 30s, and so on and so forth - so sometimes to understand what somebody is talking about in the 70s, you have to go back to the original articles and fanzines and stuff from the 50s or 30s to figure out what they mean when they use a particular term or approach...and so you build up not just a better of the original subject from the 30s, but how that subject has been handled and talked about since the 1930s.