When we talk about an artist's "peak" it seems to me that we are not talking about the artist, we are talking about us. I've seen it happen over and over. Artists change, but I don't. They see themselves and "growing", "developing", "maturing", etc. I am willing to grant them that. But too often they outgrow me. Happened with Dylan, Baez and even the Beatles. It is very rare for me to change in sync with the artist. I can name only a very few living musicians I am still listening to and eagerly awaiting output from after 25 or 30 years, e.g., Robin and Linda Williams, Mike Seeger, Martin Carthy, and a few others come to mind. But they were "niche" musicians to start with and still are. But they are niches I have stayed with.