A beaker full of death wrote:
the public usually found spin-offs to be inferior...
Usually, yes. But let's not forget such spin-offs as
The Andy Griffith Show
Gomer Pyle USMC (a spin-off of a spin-off)
The Bionic Woman
Mork & Mindy
LaVerne & Shirley
Happy Days itself
Jake and the Fatman
I dunno if The Andy Griffith Show can really be considered a spin-off from the Danny Thomas Show, since the episode in question was what they called a "backdoor pilot", much like Assignment: Earth was on Star Trek.