Jack White is the former lead guitarist/singer for the band The White Stripes. Read more about him with a quick search on Google or Wikipedia.

I think the closest word that would fit would be 'meaningful'. So, by artistically relevant I mean Elton, Paul, Billy are no longer contributing artistically in a meaningful way.

The trouble is I think they reached a resting point in their older age, revisiting songs night after night for audience appeal. Resting points, revisiting, and repeating are against the creative journey in the long run. Creative people are constantly on the move with ideas which they constantly act upon. Someone else here could probably explain it much clearer and with fewer words. I hope it makes some sense.