Provia and Velvia will fade even in archivally-safe dark storage but it has been said that they will last if projected frequently, which is not a common thing today; it is a pecularity of E6 being less archivally stable to a point. I have quite a few RVP images from 1993 which are noticeably weaker than more recent images as a visual comparison.

Kodachrome's problem was that reds were susceptible to fading more rapidly than other colours through frequent projection, and that projection was recommended for a few seconds at most. I think Kodak had a publication about this in Kodachrome's time. There is no fading problem that I can see with Kodachrome trannies from 1977 to 1992 in archival stoage. I stopped using Kodachrome around 1994.