"Yes, Adams' craft improved but, by his own admission, his artistic vision stagnated. And that's very unfortunate."
If you take a look at what are generally considered his finest images they correlate to a love interest in his life. In the various biographies you get bits & pieces of the life of Ansel & the enthusiasm of a love interest seems to help the creativity in his photographs.
Edward Weston always seemed to have a new love interest & maybe it i that excitement we see in many of his images? I don't know about Brett.