Well, a lot of the problem with these shows was lack of exposure. I mean, I don’t think Firefly ever got advertised after it debuted, with the exception of the final episode, which was, for whatever reason, heavily advertised. Yeah, it was Fox, what do you expect? Anyway, in the case of Sifl and Olly, everyone that I forced to watch it ended up loving the show and taping every episode, but it seemed that people just weren’t willing to take a chance on a show with sock puppets…
The only truly great show I can think of that has lasted for a really long time is The Simpsons. That’s because in order to last that long, things in a normal “great” show need to happen due to actual time passing and making things believable. Part of the simpson’s surrealistic charm is that for the most part, they don’t, and they are aware of this fact and exploit it to its comic utmost.
On the other hand, in the real world you are stuck in a dilemma. For instance, i’m certain the situation in most cliques would have changed drastically more over the years than, say, in “don’t go th”…er, I mean “Friends”. Certainly one of the things that makes that show mediocre is its mind-boggling constancy. I was accidentally exposed to it after an 8-year hiatus, and the plots were the same but I literally couldnt recognize the actors.
However, if you DO make changes, you risk alienating your fan base. Most shows that TRY to make realistic changes fail artistically, or at least, lose ratings.
I, for one, am guilty of being an alienated-fanbase. As a 20-something male fantasy-scifi type, I was prime clientele for Dark Angel, and watched most of the first season.
It was my favorite all-time live action show (and third behind the Simpsons and Gargoyles.) All the elements were there, I even liked the cheesy dialog, as they obviously didnt take themselves too seriously, and sometimes cheesy can become good if done with sufficient enthusiasm.
But then they tried to “move forward” with the idiotic conspiracy theory Big Plot, and I, too, “moved forward” to other series 
No, you’re on to something here, except I think you’ve got it backwards. “Frasier” is about a man and his brother who want to be subtle and sophisticated, try so very hard to be subtle and sophisticated, but end up being ridiculous instead. It’s the conflict between their delusions of grandeur and the reality they inhabit that provides the show’s greatest humor.
Everytime Frasier and Nyles attempt another social-climbing scheme, some unforseen (and very funny) piece of happenstance combines to foil them again, leaving both filled with impotent rage that thdeir superiority goes unrecognized. The show taps into a deep cultural propensity of Western European societies to satirize the wealthy.