1) The best sitcoms have a lifespan of about 7 years; for drama shows, it's about 5 years.
Representative examples: The Mary Tyler Moore Show (7 seasons exactly); The Bob Newhart Show (6 years); "Hill Street Blues" went downhill after season 5 and "M*A*S*H" had lost most of its edge by season 8.
2) At the start of a series, the actor becomes the character; during the latter days, the character conforms to the actor.
Representative examples: "M*A*S*H" again, with Hawkeye's character gradually taking on more and more of Alan Alda's personality; "Happy Days," which ended its too-long run (see #1) with most of the cast no longer bothering to conform to the fashions and hairstyles of the period.
3) Most TV show creators have one hit show in them.
Sherwood Schwartz ("Gilligan's Island," "The Brady Bunch") once said you need to have two hits -- the second one to pay for litigation for your share of the profits of the first. Unfortunately, most producers aren't that lucky.
For every Gary Marshall ("Happy Days," "Laverne & Shirley," "Mork & Mindy") and Steven Bochco ("Hill Street Blues," "L.A. Law," "NYPD Blue"), there are dozens of producers with really, one enduring show to the their name like Diane English ("Murphy Brown" and, um, "Double Rush," "Love & War" and "Ink") and David Kohan/Max Mutchnick ("Will & Grace" and uh, "Boston Common," "Four Kings" and "Good Morning Miami").
It's just not as easy as producers like Aaron Spelling (and you know his shows) make it look.