But are they producing their best work?

Perhaps rock musics seam has been competely mined out. The ability to find the excitement of doing something for the first time becomes limited. I would suggest that many of these artists with long careers lost track (mainly in the 80s) trying to follow modern trends to which they were unsuited. When people talk about their new work in glowing terms its not because its their most original, but that they have returned to producing memorable melodies played by real instruments. That is, their core strengths.