I've been listening to a lot of How Did This Get Made? and I heard one of the guest critics use the phrase "Marlon Brando Disease." That perfectly sums up what happens when a great actor completely gives up on any rational acting method and plays terrible role after terrible role.
There are obvious examples: Robert DeNiro has a relatively mild case, making occasional wtf choices like Rocky And Bullwinkle. Nicolas Cage (seriously, he was a great actor up until the 2000s) is another poster child. Johnny Depp seems to be heading that way. Leo DiCaprio is avoiding it by taking a break.
Some have it so bad that it's become almost impossible to remember that they once had promise. Val Kilmer had a light, funny touch in Top Secret and Real Genius (and was sexy as hell in Top Gun) but he's a joke now. Mickey Rourke had a dangerous charm in Angel Heart and Pope Of Greenwich Village. Hell, even Gary funking Busey was phenomenal in The Buddy Holly Story.
The disease mostly affects men in middle age (although you could make a case for Nicole Kidman). Some are immune - Michael Caine and
Dustin Hoffman Samuel L Jackson have kept their dignity in spite of Jaws The Revenge and Sphere respectively, primarily by appearing in many, many movies. Gene Hackman retired, but not before doing Welcome To Mooseport.
But the worst case by far is Al Pacino. My god... Watch Godfather, Serpico or Dog Day Afternoon and consider that that guy will go on to play a version of himself obsessed with Adam Sandler in drag.
Whose career trajectories make you weep?