Re the definition of what is obsolete, that is simple. Every concept is built based on some data. When new data have been obtained, the concept must either accommodate these data or explain why these data can be ignored. The concept that fail to do so is deemed obsolete. For instance, the early theory of the Universe claimed that the Universe is stationary. However, after discovery of the red shift by Hubble it became obvious that this theory is inconsistent with these data. Now this theory is believed to be obsolete.
The term "archival revolution" is used by scholars to describe massive opening of formerly secret archives before western researchers. Obviously, the concepts that existed before this archival revolution, and the concepts that failed to take into account new data (accept of convincingly refute) are deemed obsolete now.