Why does a field of study change? For example, we could say that something like the study of Mozart shouldn't really change, because Mozart himself doesn't. There can be new discoveries of manuscripts, but beyond that, why do we need to do research on something like that?
With scientific fields that progress in linear fashion, we can see easily that yesterday's genetics will be different from today's, but what about the Humantities. Don't we already have the Humanities? As people used to say, do we really need more interpretations of Milton or Cervantes?
In the Humanities, we have change without progress. In other words, we cannot necessarily say that we see things more clearly now than in the past. Yet we cannot remain content with past interpretations. Why is this the case? As Gadamer explained in Truth and Method, our relationship to the past is constantly shifting. 19th-century views of Shakespeare don't tell us what we want to know about Shakespeare any more.
It is very dangerous to have a literature professor who is not an active literary critic, because that person is not engaged in the process of thinking out new ways of approaching literature. Once the process stops, the reading of the work is ossified, inert.