This list certainly betrays the age of the posters! I think that Led Zeppelin is frequently mentioned because Robert Plant had the good sense to stop before they weren't very good anymore. If I were to list artists, it would be "early" Springsteen, "early" Graham Parker, etc. Dylan is one of the few exceptions, who has made great albums for a very long time.
Randy Newman wrote about this issue in the song "I'm Dead (But I Don't Know It)" in 1999! Yet most artists keep soldiering on. It's not for me to say whether or not they should keep going, but often the recent work is a pale shadow of their creative peak.