I never expect a favorite artist’s most recent album to be her best. When I count somebody among my favorites, it’s because something he released years ago made a huge impact on me. Maybe a band put out a string of excellent work in the early 90s, enough to cement them as legends, but after their heyday they settle into groove of sustained quality that falls just short of that peak.
I don’t think Elvis Costello will release a new album that I rank higher than the two or three I consider his finest. I’m sure R.E.M.’s best work is behind them, even though I’m excited as hell about their newest release. It’s reasonable to accept that musicians hit a high point and then spend the rest of their careers trying but never quite approaching it again.