Some TV shows have a structure such that you can tell almost from the beginning how long the show has to live. As much as I loved "Scrubs", that show was built around a few core characters, and once their stories were played out, that was pretty much be the end of that show. "Cheers" basically had a double length of life due to the fact it was actually two shows instead of one - the first 5 seasons with Diane and the last 6 seasons with Rebecca as the female lead. "ER" is different. It has a large cast of constantly revolving characters, and the story lines will always be there as long as there is controversy in medicine to merge with the personal drama. Early in ER's history, things were different. George Clooney's character, Doug Ross, was really the star of the show, although they did spread the stories around so that there was quite a bit of focus on the other characters too. This was a successful formula, but once Clooney became a star and a heartthrob he quickly tired of television and longed for the big screen. Thus, starting in season four, he is absent more and more as he goes off to make action films and the show began to look like it was going to suffer from "Welcome Back Kotter" syndrome, where John Travolta's success on the silver screen killed that show. After Clooney actually did formally exit stage left, the show changed the formula to its current one of spreading the action around with nobody in particular having the spotlight. I guess my point with all of this is, this is how ER managed to go on a total of 15 seasons, with even one extremely unlikeable character being written in as interesting, even if that one character in particular came to an end worthy of Wiley Coyote.