Which phrase comes from 'Happy Days' and means a show has passed its peak?

In 'Downton Abbey', what is the family's surname? In 'Dallas', what is the name of the scheming oil baron? What was the famous cliffhanger question from 'Dallas' in 1980? In 'Happy Days', what is Fonzie's full name? In 'I Love Lucy', who played Lucy Ricardo? In 'I Love Lucy', what is the name of Lucy's husband? In 'The Andy Griffith Show', in which fictional town is the show set? In 'M*A*S*H', during which war is the series set?