Laverne and Shirley Jumped the Shark...
...when the gang moved to sunny California. I had no desire to move along with them. The dingy Milwaukee basement apartment was it's own character, similar to the Adam-12 radio car being a character unto itself, per Jack Webb. It helped define the duo.
... when Lenny and Squiggy stopped being juvenile-minded, somewhat creepy, lecherous young men of Milwaukee. The less creepy but still horned-up doofus duo of Lenny and Squiggy became zany caricatures.