Jerry Lundegaard had a good job at his father in law’s dealership, and a nice new house in the 'burbs. Then he made some ill-advised decisions.
Comedies?
It’s a Mad, Mad, Mad, Mad World. All of the main male characters go from respected citizens to being terribly injured and facing trial, with Spencer Tracy being the worst off of all.
Buster Keaton in One Week. Buster decides to build his own home from a kit, and things go from bad to worse.
Tyrone Power in Nightmare Alley.
Matthew Broderick in Election
Eric Packer in Cosmopolis.
Just the one I was coming in to mention. Matthew Broderick’s character throws away his successful career and happy marriage, without being more than a speed-bump in Tracy Flick’s path in life (the novel is somewhat different)
I can’t remember the title but it’s one of the first movies ever made but it’s about a haughty matre’d of a hotel whose total snob and ends up getting demoted down to restroom attendant … I think he ends his own life over it …
I ruled out The Last Laugh in post 7: his downfall wasn’t his fault and he made a comeback in the end.
The actor who played him, Emil Jannings, does fit: his German accent killed his move to Hollywood at the advent of talkies, so he went back home and made movies for Goebbels.
As much as I hate to say it, Make Way for Tomorrow qualifies for the thread. They expected to the very end that they could keep their home, or move in together with their grown children, so they turned down the offer to be the caretaker couple on the farm.
Trivia: he was the first Academy Award winner for best male actor in 1929.
Not the full movie but a chapter from Woody Allen’s Everything You Always Wanted to Know About Sex But Were Afraid to Ask. Gene Wilder plays a successful and well respected physician who’s lust for a sheep named Daisy lands him in the gutter drowning his sorrows with a bottle of Woolite.
Just thought of one, Peter Berg’s character in The Last Seduction. Actually, I think this was the character arc of the male lead in almost every 90’s Linda Fiorentino movie.
William Hurt’s character in Body Heat.
The lead character in I Served the King of England.
Stan is a drifter or a Carnival Barker. That is very very low on the scale of Prosperity. And in the original there is a ray of hope at the very end.
The remake really telegraphs the ending. Guillermo del Toro wasn’t subtle there. The original was better (and less smoking too) 
Googling, though, the dark ending of the new version of Nightmare Alley is consistent with the original novel, unlike the slightly upbeat ending of the 1947 film version.
I can believe that.
And it was just a ray of hope, not quite a happy ending.
Ethan Frome
Ethan is in a fraught relationship with his hypochondriac wife Zeena. He falls in love with Mattie, a young woman who has come to stay with the Fromes. Ethan & Mattie wish to do a runner together but have no money, so they decide on a suicide pact which goes wrong and results in a somewhat depressing ending.
Ethan is left with a permanent limp but Mattie is paralysed and:
“In an agonizing irony, Ethan and Mattie have gotten their wish to stay together, but in mutual unhappiness and discontent, with Mattie helpless and paralyzed, and with Zeena as a constant presence between the two of them.”
I think it’s safe to say that women often feature prominently when a man goes down the tubes.
If I think I have this right - it’s poor, then rich, then poor again, and then rich one more time, in The Jerk.
He was born a poor black child.