This is a really good topic, IMO.
I can think of long running bands that were amazing when they started and are fantastic today, but had a slump, meaning a time when they were not so amazing. The best example of this is Depeche Mode: while I think Exciter and Ultra were good albums, not even the most hardcore DM fan can say they were on the same level as Music For The Masses, Black Celebration, Violator, or even Songs Of Faith And Devotion, and they certainly don't touch DM's
newest release, Playing The Angel, which, far from the stereotypical "return to form", is actually the creation of an entirely new form. 25 year old bands aren't supposed to make albums like Angel. Freaking amazing.
Hmmm, Tom Waits might be a truer answer to the OP. He released a lot of excellent albums early-mid career, but it is arguable that his best works were his 90's albums he wrote years after he sobered up: Alice, Mule Variations, etc. I haven't heard his latest, Orphans, but the critics are having a rock aneurysm over it.