The ultimate safe-bet/surefire-fail paradox in TV programming is the spinoff. Usually, they make a big splash, only to fade away (see #1, #3 and especially #2), but sometimes they outlive their parent, in both longevity and in popularity (#4).
#1 - Mork & Mindy - Spun off from Happy Days. This is the extended version of season two's opening (rarely seen on TV today). Season two just about killed the show, after a stellar first year. They changed time slots and most of the cast. Hey, good going.
#2 - Joanie Loves Chachi - Also spun off (and back onto) Happy Days. Very popular at first, then *poof!* I guess it was especially successful in
#3 - Rhoda - Spun off from The Mary Tyler Moore Show. Very popular, until Rhoda got married, then sank in the ratings. And poor Valerie Harper--an underrated comic talent--never really got that next big break.
#4 - The Facts of Life - Spun off from the abysmal Diff'rent Strokes, this show may have been slightly vapid, but hey, it had George Clooney for a while! And that hobbit guy's brother!
Relive the magic. . .