UX, as a term, didn't really exist in the 1990s: https://books.google.com/ngrams/graph?content=user+interface...
That's consistent with your timeline of the decline of UI/UX though. My sense is that the birth of the term UX marked the beginning of the decline because it meant redefining the term UI as being purely about aesthetics, implying that no one was paying attention to all of the non-aesthetic work that had previously been done in the field.